2156 - split edit.mu into multiple files

Now you can bring up the programming environment by saying:

  $ mu edit

The files under edit aren't yet *layers*, though, they have a few
dependencies that we need to clean up.

12 files changed, 8903 insertions, 0 deletions

diff --git a/edit/001-editor.mu b/edit/001-editor.mu
new file mode 100644
index 00000000..8909a574
--- /dev/null
+++ b/edit/001-editor.mu
@@ -0,0 +1,625 @@
+## the basic editor data structure, and how it displays text to the screen
+
+scenario editor-initially-prints-string-to-screen [
+  assume-screen 10/width, 5/height
+  run [
+    1:address:array:character <- new [abc]
+    new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .          .
+  ]
+]
+
+container editor-data [
+  # editable text: doubly linked list of characters (head contains a special sentinel)
+  data:address:duplex-list:character
+  top-of-screen:address:duplex-list:character
+  bottom-of-screen:address:duplex-list:character
+  # location before cursor inside data
+  before-cursor:address:duplex-list:character
+
+  # raw bounds of display area on screen
+  # always displays from row 1 (leaving row 0 for a menu) and at most until bottom of screen
+  left:number
+  right:number
+  # raw screen coordinates of cursor
+  cursor-row:number
+  cursor-column:number
+]
+
+# editor:address, screen <- new-editor s:address:array:character, screen:address, left:number, right:number
+# creates a new editor widget and renders its initial appearance to screen.
+#   top/left/right constrain the screen area available to the new editor.
+#   right is exclusive.
+recipe new-editor [
+  local-scope
+  s:address:array:character <- next-ingredient
+  screen:address <- next-ingredient
+  # no clipping of bounds
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  right <- subtract right, 1
+  result:address:editor-data <- new editor-data:type
+  # initialize screen-related fields
+  x:address:number <- get-address *result, left:offset
+  *x <- copy left
+  x <- get-address *result, right:offset
+  *x <- copy right
+  # initialize cursor
+  x <- get-address *result, cursor-row:offset
+  *x <- copy 1/top
+  x <- get-address *result, cursor-column:offset
+  *x <- copy left
+  init:address:address:duplex-list <- get-address *result, data:offset
+  *init <- push-duplex 167/§, 0/tail
+  top-of-screen:address:address:duplex-list <- get-address *result, top-of-screen:offset
+  *top-of-screen <- copy *init
+  y:address:address:duplex-list <- get-address *result, before-cursor:offset
+  *y <- copy *init
+  result <- insert-text result, s
+  # initialize cursor to top of screen
+  y <- get-address *result, before-cursor:offset
+  *y <- copy *init
+  # initial render to screen, just for some old tests
+  _, _, screen, result <- render screen, result
+  <editor-initialization>
+  reply result
+]
+
+recipe insert-text [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  text:address:array:character <- next-ingredient
+  # early exit if text is empty
+  reply-unless text, editor/same-as-ingredient:0
+  len:number <- length *text
+  reply-unless len, editor/same-as-ingredient:0
+  idx:number <- copy 0
+  # now we can start appending the rest, character by character
+  curr:address:duplex-list <- get *editor, data:offset
+  {
+    done?:boolean <- greater-or-equal idx, len
+    break-if done?
+    c:character <- index *text, idx
+    insert-duplex c, curr
+    # next iter
+    curr <- next-duplex curr
+    idx <- add idx, 1
+    loop
+  }
+  reply editor/same-as-ingredient:0
+]
+
+scenario editor-initializes-without-data [
+  assume-screen 5/width, 3/height
+  run [
+    1:address:editor-data <- new-editor 0/data, screen:address, 2/left, 5/right
+    2:editor-data <- copy *1:address:editor-data
+  ]
+  memory-should-contain [
+    # 2 (data) <- just the § sentinel
+    # 3 (top of screen) <- the § sentinel
+    4 <- 0  # bottom-of-screen; null since text fits on screen
+    # 5 (before cursor) <- the § sentinel
+    6 <- 2  # left
+    7 <- 4  # right  (inclusive)
+    8 <- 1  # cursor row
+    9 <- 2  # cursor column
+  ]
+  screen-should-contain [
+    .     .
+    .     .
+    .     .
+  ]
+]
+
+# last-row:number, last-column:number, screen, editor <- render screen:address, editor:address:editor-data
+#
+# Assumes cursor should be at coordinates (cursor-row, cursor-column) and
+# updates before-cursor to match. Might also move coordinates if they're
+# outside text.
+recipe render [
+  local-scope
+  screen:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  reply-unless editor, 1/top, 0/left, screen/same-as-ingredient:0, editor/same-as-ingredient:1
+  left:number <- get *editor, left:offset
+  screen-height:number <- screen-height screen
+  right:number <- get *editor, right:offset
+  # traversing editor
+  curr:address:duplex-list <- get *editor, top-of-screen:offset
+  prev:address:duplex-list <- copy curr  # just in case curr becomes null and we can't compute prev-duplex
+  curr <- next-duplex curr
+  # traversing screen
+  +render-loop-initialization
+  color:number <- copy 7/white
+  row:number <- copy 1/top
+  column:number <- copy left
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  screen <- move-cursor screen, row, column
+  {
+    +next-character
+    break-unless curr
+    off-screen?:boolean <- greater-or-equal row, screen-height
+    break-if off-screen?
+    # update editor-data.before-cursor
+    # Doing so at the start of each iteration ensures it stays one step behind
+    # the current character.
+    {
+      at-cursor-row?:boolean <- equal row, *cursor-row
+      break-unless at-cursor-row?
+      at-cursor?:boolean <- equal column, *cursor-column
+      break-unless at-cursor?
+      *before-cursor <- copy prev
+    }
+    c:character <- get *curr, value:offset
+    <character-c-received>
+    {
+      # newline? move to left rather than 0
+      newline?:boolean <- equal c, 10/newline
+      break-unless newline?
+      # adjust cursor if necessary
+      {
+        at-cursor-row?:boolean <- equal row, *cursor-row
+        break-unless at-cursor-row?
+        left-of-cursor?:boolean <- lesser-than column, *cursor-column
+        break-unless left-of-cursor?
+        *cursor-column <- copy column
+        *before-cursor <- prev-duplex curr
+      }
+      # clear rest of line in this window
+      clear-line-delimited screen, column, right
+      # skip to next line
+      row <- add row, 1
+      column <- copy left
+      screen <- move-cursor screen, row, column
+      curr <- next-duplex curr
+      prev <- next-duplex prev
+      loop +next-character:label
+    }
+    {
+      # at right? wrap. even if there's only one more letter left; we need
+      # room for clicking on the cursor after it.
+      at-right?:boolean <- equal column, right
+      break-unless at-right?
+      # print wrap icon
+      print-character screen, 8617/loop-back-to-left, 245/grey
+      column <- copy left
+      row <- add row, 1
+      screen <- move-cursor screen, row, column
+      # don't increment curr
+      loop +next-character:label
+    }
+    print-character screen, c, color
+    curr <- next-duplex curr
+    prev <- next-duplex prev
+    column <- add column, 1
+    loop
+  }
+  # save first character off-screen
+  bottom-of-screen:address:address:duplex-list <- get-address *editor, bottom-of-screen:offset
+  *bottom-of-screen <- copy curr
+  # is cursor to the right of the last line? move to end
+  {
+    at-cursor-row?:boolean <- equal row, *cursor-row
+    cursor-outside-line?:boolean <- lesser-or-equal column, *cursor-column
+    before-cursor-on-same-line?:boolean <- and at-cursor-row?, cursor-outside-line?
+    above-cursor-row?:boolean <- lesser-than row, *cursor-row
+    before-cursor?:boolean <- or before-cursor-on-same-line?, above-cursor-row?
+    break-unless before-cursor?
+    *cursor-row <- copy row
+    *cursor-column <- copy column
+    *before-cursor <- copy prev
+  }
+  reply row, column, screen/same-as-ingredient:0, editor/same-as-ingredient:1
+]
+
+# row, screen <- render-string screen:address, s:address:array:character, left:number, right:number, color:number, row:number
+# move cursor at start of next line
+# print a string 's' to 'editor' in 'color' starting at 'row'
+# clear rest of last line, but don't move cursor to next line
+recipe render-string [
+  local-scope
+  screen:address <- next-ingredient
+  s:address:array:character <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  color:number <- next-ingredient
+  row:number <- next-ingredient
+  row <- add row, 1
+  reply-unless s, row/same-as-ingredient:5, screen/same-as-ingredient:0
+  column:number <- copy left
+  screen <- move-cursor screen, row, column
+  screen-height:number <- screen-height screen
+  i:number <- copy 0
+  len:number <- length *s
+  {
+    +next-character
+    done?:boolean <- greater-or-equal i, len
+    break-if done?
+    done? <- greater-or-equal row, screen-height
+    break-if done?
+    c:character <- index *s, i
+    {
+      # at right? wrap.
+      at-right?:boolean <- equal column, right
+      break-unless at-right?
+      # print wrap icon
+      print-character screen, 8617/loop-back-to-left, 245/grey
+      column <- copy left
+      row <- add row, 1
+      screen <- move-cursor screen, row, column
+      loop +next-character:label  # retry i
+    }
+    i <- add i, 1
+    {
+      # newline? move to left rather than 0
+      newline?:boolean <- equal c, 10/newline
+      break-unless newline?
+      # clear rest of line in this window
+      {
+        done?:boolean <- greater-than column, right
+        break-if done?
+        print-character screen, 32/space
+        column <- add column, 1
+        loop
+      }
+      row <- add row, 1
+      column <- copy left
+      screen <- move-cursor screen, row, column
+      loop +next-character:label
+    }
+    print-character screen, c, color
+    column <- add column, 1
+    loop
+  }
+  {
+    # clear rest of current line
+    line-done?:boolean <- greater-than column, right
+    break-if line-done?
+    print-character screen, 32/space
+    column <- add column, 1
+    loop
+  }
+  reply row/same-as-ingredient:5, screen/same-as-ingredient:0
+]
+
+# row, screen <- render-code-string screen:address, s:address:array:character, left:number, right:number, row:number
+# like 'render-string' but with colorization for comments like in the editor
+recipe render-code-string [
+  local-scope
+  screen:address <- next-ingredient
+  s:address:array:character <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  row:number <- next-ingredient
+  row <- add row, 1
+  reply-unless s, row/same-as-ingredient:4, screen/same-as-ingredient:0
+  color:number <- copy 7/white
+  column:number <- copy left
+  screen <- move-cursor screen, row, column
+  screen-height:number <- screen-height screen
+  i:number <- copy 0
+  len:number <- length *s
+  {
+    +next-character
+    done?:boolean <- greater-or-equal i, len
+    break-if done?
+    done? <- greater-or-equal row, screen-height
+    break-if done?
+    c:character <- index *s, i
+    <character-c-received>  # only line different from render-string
+    {
+      # at right? wrap.
+      at-right?:boolean <- equal column, right
+      break-unless at-right?
+      # print wrap icon
+      print-character screen, 8617/loop-back-to-left, 245/grey
+      column <- copy left
+      row <- add row, 1
+      screen <- move-cursor screen, row, column
+      loop +next-character:label  # retry i
+    }
+    i <- add i, 1
+    {
+      # newline? move to left rather than 0
+      newline?:boolean <- equal c, 10/newline
+      break-unless newline?
+      # clear rest of line in this window
+      {
+        done?:boolean <- greater-than column, right
+        break-if done?
+        print-character screen, 32/space
+        column <- add column, 1
+        loop
+      }
+      row <- add row, 1
+      column <- copy left
+      screen <- move-cursor screen, row, column
+      loop +next-character:label
+    }
+    print-character screen, c, color
+    column <- add column, 1
+    loop
+  }
+  {
+    # clear rest of current line
+    line-done?:boolean <- greater-than column, right
+    break-if line-done?
+    print-character screen, 32/space
+    column <- add column, 1
+    loop
+  }
+  reply row/same-as-ingredient:4, screen/same-as-ingredient:0
+]
+
+recipe clear-line-delimited [
+  local-scope
+  screen:address <- next-ingredient
+  column:number <- next-ingredient
+  right:number <- next-ingredient
+  {
+    done?:boolean <- greater-than column, right
+    break-if done?
+    print-character screen, 32/space
+    column <- add column, 1
+    loop
+  }
+]
+
+recipe clear-screen-from [
+  local-scope
+  screen:address <- next-ingredient
+  row:number <- next-ingredient
+  column:number <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  # if it's the real screen, use the optimized primitive
+  {
+    break-if screen
+    clear-display-from row, column, left, right
+    reply screen/same-as-ingredient:0
+  }
+  # if not, go the slower route
+  screen <- move-cursor screen, row, column
+  clear-line-delimited screen, column, right
+  clear-rest-of-screen screen, row, left, right
+  reply screen/same-as-ingredient:0
+]
+
+recipe clear-rest-of-screen [
+  local-scope
+  screen:address <- next-ingredient
+  row:number <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  row <- add row, 1
+  screen <- move-cursor screen, row, left
+  screen-height:number <- screen-height screen
+  {
+    at-bottom-of-screen?:boolean <- greater-or-equal row, screen-height
+    break-if at-bottom-of-screen?
+    screen <- move-cursor screen, row, left
+    clear-line-delimited screen, left, right
+    row <- add row, 1
+    loop
+  }
+]
+
+scenario editor-initially-prints-multiple-lines [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abc
+def]
+    new-editor s:address:array:character, screen:address, 0/left, 5/right
+  ]
+  screen-should-contain [
+    .     .
+    .abc  .
+    .def  .
+    .     .
+  ]
+]
+
+scenario editor-initially-handles-offsets [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abc]
+    new-editor s:address:array:character, screen:address, 1/left, 5/right
+  ]
+  screen-should-contain [
+    .     .
+    . abc .
+    .     .
+  ]
+]
+
+scenario editor-initially-prints-multiple-lines-at-offset [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abc
+def]
+    new-editor s:address:array:character, screen:address, 1/left, 5/right
+  ]
+  screen-should-contain [
+    .     .
+    . abc .
+    . def .
+    .     .
+  ]
+]
+
+scenario editor-initially-wraps-long-lines [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abc def]
+    new-editor s:address:array:character, screen:address, 0/left, 5/right
+  ]
+  screen-should-contain [
+    .     .
+    .abc ↩.
+    .def  .
+    .     .
+  ]
+  screen-should-contain-in-color 245/grey [
+    .     .
+    .    ↩.
+    .     .
+    .     .
+  ]
+]
+
+scenario editor-initially-wraps-barely-long-lines [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abcde]
+    new-editor s:address:array:character, screen:address, 0/left, 5/right
+  ]
+  # still wrap, even though the line would fit. We need room to click on the
+  # end of the line
+  screen-should-contain [
+    .     .
+    .abcd↩.
+    .e    .
+    .     .
+  ]
+  screen-should-contain-in-color 245/grey [
+    .     .
+    .    ↩.
+    .     .
+    .     .
+  ]
+]
+
+scenario editor-initializes-empty-text [
+  assume-screen 5/width, 5/height
+  run [
+    1:address:array:character <- new []
+    2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .     .
+    .     .
+    .     .
+  ]
+  memory-should-contain [
+    3 <- 1  # cursor row
+    4 <- 0  # cursor column
+  ]
+]
+
+# just a little color for mu code
+
+scenario render-colors-comments [
+  assume-screen 5/width, 5/height
+  run [
+    s:address:array:character <- new [abc
+# de
+f]
+    new-editor s:address:array:character, screen:address, 0/left, 5/right
+  ]
+  screen-should-contain [
+    .     .
+    .abc  .
+    .# de .
+    .f    .
+    .     .
+  ]
+  screen-should-contain-in-color 12/lightblue, [
+    .     .
+    .     .
+    .# de .
+    .     .
+    .     .
+  ]
+  screen-should-contain-in-color 7/white, [
+    .     .
+    .abc  .
+    .     .
+    .f    .
+    .     .
+  ]
+]
+
+after <character-c-received> [
+  color <- get-color color, c
+]
+
+# color <- get-color color:number, c:character
+# so far the previous color is all the information we need; that may change
+recipe get-color [
+  local-scope
+  color:number <- next-ingredient
+  c:character <- next-ingredient
+  color-is-white?:boolean <- equal color, 7/white
+  # if color is white and next character is '#', switch color to blue
+  {
+    break-unless color-is-white?
+    starting-comment?:boolean <- equal c, 35/#
+    break-unless starting-comment?
+    trace 90, [app], [switch color back to blue]
+    color <- copy 12/lightblue
+    jump +exit:label
+  }
+  # if color is blue and next character is newline, switch color to white
+  {
+    color-is-blue?:boolean <- equal color, 12/lightblue
+    break-unless color-is-blue?
+    ending-comment?:boolean <- equal c, 10/newline
+    break-unless ending-comment?
+    trace 90, [app], [switch color back to white]
+    color <- copy 7/white
+    jump +exit:label
+  }
+  # if color is white (no comments) and next character is '<', switch color to red
+  {
+    break-unless color-is-white?
+    starting-assignment?:boolean <- equal c, 60/<
+    break-unless starting-assignment?
+    color <- copy 1/red
+    jump +exit:label
+  }
+  # if color is red and next character is space, switch color to white
+  {
+    color-is-red?:boolean <- equal color, 1/red
+    break-unless color-is-red?
+    ending-assignment?:boolean <- equal c, 32/space
+    break-unless ending-assignment?
+    color <- copy 7/white
+    jump +exit:label
+  }
+  # otherwise no change
+  +exit
+  reply color
+]
+
+scenario render-colors-assignment [
+  assume-screen 8/width, 5/height
+  run [
+    s:address:array:character <- new [abc
+d <- e
+f]
+    new-editor s:address:array:character, screen:address, 0/left, 8/right
+  ]
+  screen-should-contain [
+    .        .
+    .abc     .
+    .d <- e  .
+    .f       .
+    .        .
+  ]
+  screen-should-contain-in-color 1/red, [
+    .        .
+    .        .
+    .  <-    .
+    .        .
+    .        .
+  ]
+]
diff --git a/edit/002-typing.mu b/edit/002-typing.mu
new file mode 100644
index 00000000..717c630e
--- /dev/null
+++ b/edit/002-typing.mu
@@ -0,0 +1,1012 @@
+## handling events from the keyboard, mouse, touch screen, ...
+
+recipe editor-event-loop [
+  local-scope
+  screen:address <- next-ingredient
+  console:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  {
+    # looping over each (keyboard or touch) event as it occurs
+    +next-event
+    e:event, console:address, found?:boolean, quit?:boolean <- read-event console
+    loop-unless found?
+    break-if quit?  # only in tests
+    trace 10, [app], [next-event]
+    # 'touch' event
+    t:address:touch-event <- maybe-convert e, touch:variant
+    {
+      break-unless t
+      move-cursor-in-editor screen, editor, *t
+      loop +next-event:label
+    }
+    # keyboard events
+    {
+      break-if t
+      screen, editor, go-render?:boolean <- handle-keyboard-event screen, editor, e
+      {
+        break-unless go-render?
+        editor-render screen, editor
+      }
+    }
+    loop
+  }
+]
+
+# process click, return if it was on current editor
+recipe move-cursor-in-editor [
+  local-scope
+  screen:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  t:touch-event <- next-ingredient
+  reply-unless editor, 0/false
+  click-row:number <- get t, row:offset
+  reply-unless click-row, 0/false  # ignore clicks on 'menu'
+  click-column:number <- get t, column:offset
+  left:number <- get *editor, left:offset
+  too-far-left?:boolean <- lesser-than click-column, left
+  reply-if too-far-left?, 0/false
+  right:number <- get *editor, right:offset
+  too-far-right?:boolean <- greater-than click-column, right
+  reply-if too-far-right?, 0/false
+  # position cursor
+  <move-cursor-begin>
+  editor <- snap-cursor screen, editor, click-row, click-column
+  undo-coalesce-tag:number <- copy 0/never
+  <move-cursor-end>
+  # gain focus
+  reply 1/true
+]
+
+# editor <- snap-cursor screen:address, editor:address:editor-data, target-row:number, target-column:number
+#
+# Variant of 'render' that only moves the cursor (coordinates and
+# before-cursor). If it's past the end of a line, it 'slides' it left. If it's
+# past the last line it positions at end of last line.
+recipe snap-cursor [
+  local-scope
+  screen:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  target-row:number <- next-ingredient
+  target-column:number <- next-ingredient
+  reply-unless editor, 1/top, editor/same-as-ingredient:1
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  screen-height:number <- screen-height screen
+  # count newlines until screen row
+  curr:address:duplex-list <- get *editor, top-of-screen:offset
+  prev:address:duplex-list <- copy curr  # just in case curr becomes null and we can't compute prev-duplex
+  curr <- next-duplex curr
+  row:number <- copy 1/top
+  column:number <- copy left
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  *cursor-row <- copy target-row
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  *cursor-column <- copy target-column
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  {
+    +next-character
+    break-unless curr
+    off-screen?:boolean <- greater-or-equal row, screen-height
+    break-if off-screen?
+    # update editor-data.before-cursor
+    # Doing so at the start of each iteration ensures it stays one step behind
+    # the current character.
+    {
+      at-cursor-row?:boolean <- equal row, *cursor-row
+      break-unless at-cursor-row?
+      at-cursor?:boolean <- equal column, *cursor-column
+      break-unless at-cursor?
+      *before-cursor <- copy prev
+    }
+    c:character <- get *curr, value:offset
+    {
+      # newline? move to left rather than 0
+      newline?:boolean <- equal c, 10/newline
+      break-unless newline?
+      # adjust cursor if necessary
+      {
+        at-cursor-row?:boolean <- equal row, *cursor-row
+        break-unless at-cursor-row?
+        left-of-cursor?:boolean <- lesser-than column, *cursor-column
+        break-unless left-of-cursor?
+        *cursor-column <- copy column
+        *before-cursor <- copy prev
+      }
+      # skip to next line
+      row <- add row, 1
+      column <- copy left
+      curr <- next-duplex curr
+      prev <- next-duplex prev
+      loop +next-character:label
+    }
+    {
+      # at right? wrap. even if there's only one more letter left; we need
+      # room for clicking on the cursor after it.
+      at-right?:boolean <- equal column, right
+      break-unless at-right?
+      column <- copy left
+      row <- add row, 1
+      # don't increment curr/prev
+      loop +next-character:label
+    }
+    curr <- next-duplex curr
+    prev <- next-duplex prev
+    column <- add column, 1
+    loop
+  }
+  # is cursor to the right of the last line? move to end
+  {
+    at-cursor-row?:boolean <- equal row, *cursor-row
+    cursor-outside-line?:boolean <- lesser-or-equal column, *cursor-column
+    before-cursor-on-same-line?:boolean <- and at-cursor-row?, cursor-outside-line?
+    above-cursor-row?:boolean <- lesser-than row, *cursor-row
+    before-cursor?:boolean <- or before-cursor-on-same-line?, above-cursor-row?
+    break-unless before-cursor?
+    *cursor-row <- copy row
+    *cursor-column <- copy column
+    *before-cursor <- copy prev
+  }
+  reply editor/same-as-ingredient:1
+]
+
+# screen, editor, go-render?:boolean <- handle-keyboard-event screen:address, editor:address:editor-data, e:event
+# Process an event 'e' and try to minimally update the screen.
+# Set 'go-render?' to true to indicate the caller must perform a non-minimal update.
+recipe handle-keyboard-event [
+  local-scope
+  screen:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  e:event <- next-ingredient
+  reply-unless editor, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+  screen-width:number <- screen-width screen
+  screen-height:number <- screen-height screen
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  save-row:number <- copy *cursor-row
+  save-column:number <- copy *cursor-column
+  # character
+  {
+    c:address:character <- maybe-convert e, text:variant
+    break-unless c
+    trace 10, [app], [handle-keyboard-event: special character]
+    # exceptions for special characters go here
+    <handle-special-character>
+    # ignore any other special characters
+    regular-character?:boolean <- greater-or-equal *c, 32/space
+    reply-unless regular-character?, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    # otherwise type it in
+    <insert-character-begin>
+    editor, screen, go-render?:boolean <- insert-at-cursor editor, *c, screen
+    <insert-character-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+  # special key to modify the text or move the cursor
+  k:address:number <- maybe-convert e:event, keycode:variant
+  assert k, [event was of unknown type; neither keyboard nor mouse]
+  # handlers for each special key will go here
+  <handle-special-key>
+  reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+]
+
+recipe insert-at-cursor [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  c:character <- next-ingredient
+  screen:address <- next-ingredient
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  insert-duplex c, *before-cursor
+  *before-cursor <- next-duplex *before-cursor
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  save-row:number <- copy *cursor-row
+  save-column:number <- copy *cursor-column
+  screen-width:number <- screen-width screen
+  screen-height:number <- screen-height screen
+  # occasionally we'll need to mess with the cursor
+  <insert-character-special-case>
+  # but mostly we'll just move the cursor right
+  *cursor-column <- add *cursor-column, 1
+  next:address:duplex-list <- next-duplex *before-cursor
+  {
+    # at end of all text? no need to scroll? just print the character and leave
+    at-end?:boolean <- equal next, 0/null
+    break-unless at-end?
+    bottom:number <- subtract screen-height, 1
+    at-bottom?:boolean <- equal save-row, bottom
+    at-right?:boolean <- equal save-column, right
+    overflow?:boolean <- and at-bottom?, at-right?
+    break-if overflow?
+    move-cursor screen, save-row, save-column
+    print-character screen, c
+    reply editor/same-as-ingredient:0, screen/same-as-ingredient:2, 0/no-more-render
+  }
+  {
+    # not at right margin? print the character and rest of line
+    break-unless next
+    at-right?:boolean <- greater-or-equal *cursor-column, screen-width
+    break-if at-right?
+    curr:address:duplex-list <- copy *before-cursor
+    move-cursor screen, save-row, save-column
+    curr-column:number <- copy save-column
+    {
+      # hit right margin? give up and let caller render
+      at-right?:boolean <- greater-than curr-column, right
+      reply-if at-right?, editor/same-as-ingredient:0, screen/same-as-ingredient:2, 1/go-render
+      break-unless curr
+      # newline? done.
+      currc:character <- get *curr, value:offset
+      at-newline?:boolean <- equal currc, 10/newline
+      break-if at-newline?
+      print-character screen, currc
+      curr-column <- add curr-column, 1
+      curr <- next-duplex curr
+      loop
+    }
+    reply editor/same-as-ingredient:0, screen/same-as-ingredient:2, 0/no-more-render
+  }
+  reply editor/same-as-ingredient:0, screen/same-as-ingredient:2, 1/go-render
+]
+
+# helper for tests
+recipe editor-render [
+  local-scope
+  screen:address <- next-ingredient
+  editor:address:editor-data <- next-ingredient
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  row:number, column:number <- render screen, editor
+  clear-line-delimited screen, column, right
+  row <- add row, 1
+  draw-horizontal screen, row, left, right, 9480/horizontal-dotted
+  row <- add row, 1
+  clear-screen-from screen, row, left, left, right
+]
+
+scenario editor-handles-empty-event-queue [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console []
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-handles-mouse-clicks [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 1  # on the 'b'
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1  # cursor is at row 0..
+    4 <- 1  # ..and column 1
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-handles-mouse-clicks-outside-text [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  $clear-trace
+  assume-console [
+    left-click 1, 7  # last line, to the right of text
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1  # cursor row
+    4 <- 3  # cursor column
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-handles-mouse-clicks-outside-text-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  $clear-trace
+  assume-console [
+    left-click 1, 7  # interior line, to the right of text
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1  # cursor row
+    4 <- 3  # cursor column
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-handles-mouse-clicks-outside-text-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  $clear-trace
+  assume-console [
+    left-click 3, 7  # below text
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 2  # cursor row
+    4 <- 3  # cursor column
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-handles-mouse-clicks-outside-column [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  # editor occupies only left half of screen
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    # click on right half of screen
+    left-click 3, 8
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1  # no change to cursor row
+    4 <- 0  # ..or column
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-handles-mouse-clicks-in-menu-area [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    # click on first, 'menu' row
+    left-click 0, 3
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # no change to cursor
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+]
+
+scenario editor-inserts-characters-into-empty-editor [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    type [abc]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  check-trace-count-for-label 3, [print-character]
+]
+
+scenario editor-inserts-characters-at-cursor [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # type two letters at different places
+  assume-console [
+    type [0]
+    left-click 1, 2
+    type [d]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .0adbc     .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 7, [print-character]  # 4 for first letter, 3 for second
+]
+
+scenario editor-inserts-characters-at-cursor-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 5  # right of last line
+    type [d]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abcd      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 1, [print-character]
+]
+
+scenario editor-inserts-characters-at-cursor-5 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 5  # right of non-last line
+    type [e]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abce      .
+    .d         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 1, [print-character]
+]
+
+scenario editor-inserts-characters-at-cursor-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 3, 5  # below all text
+    type [d]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abcd      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 1, [print-character]
+]
+
+scenario editor-inserts-characters-at-cursor-4 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 3, 5  # below all text
+    type [e]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .de        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 1, [print-character]
+]
+
+scenario editor-inserts-characters-at-cursor-6 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 3, 5  # below all text
+    type [ef]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 2, [print-character]
+]
+
+scenario editor-moves-cursor-after-inserting-characters [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [ab]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [01]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .01ab      .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+]
+
+# if the cursor reaches the right margin, wrap the line
+
+scenario editor-wraps-line-on-insert [
+  assume-screen 5/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  # type a letter
+  assume-console [
+    type [e]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # no wrap yet
+  screen-should-contain [
+    .     .
+    .eabc .
+    .┈┈┈┈┈.
+    .     .
+    .     .
+  ]
+  # type a second letter
+  assume-console [
+    type [f]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # now wrap
+  screen-should-contain [
+    .     .
+    .efab↩.
+    .c    .
+    .┈┈┈┈┈.
+    .     .
+  ]
+]
+
+scenario editor-wraps-line-on-insert-2 [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcdefg
+defg]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  # type more text at the start
+  assume-console [
+    left-click 3, 0
+    type [abc]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor is not wrapped
+  memory-should-contain [
+    3 <- 3
+    4 <- 3
+  ]
+  # but line is wrapped
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .efg       .
+    .abcd↩     .
+    .efg       .
+  ]
+]
+
+after <insert-character-special-case> [
+  # if the line wraps at the cursor, move cursor to start of next row
+  {
+    # if we're at the column just before the wrap indicator
+    wrap-column:number <- subtract right, 1
+    at-wrap?:boolean <- greater-or-equal *cursor-column, wrap-column
+    break-unless at-wrap?
+    *cursor-column <- subtract *cursor-column, wrap-column
+    *cursor-row <- add *cursor-row, 1
+    # if we're out of the screen, scroll down
+    {
+      below-screen?:boolean <- greater-or-equal *cursor-row, screen-height
+      break-unless below-screen?
+      <scroll-down>
+    }
+    reply editor/same-as-ingredient:0, screen/same-as-ingredient:2, 1/go-render
+  }
+]
+
+scenario editor-wraps-cursor-after-inserting-characters [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcde]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  assume-console [
+    left-click 1, 4  # line is full; no wrap icon yet
+    type [f]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .fe        .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 2  # cursor row
+    4 <- 1  # cursor column
+  ]
+]
+
+scenario editor-wraps-cursor-after-inserting-characters-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcde]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  assume-console [
+    left-click 1, 3  # right before the wrap icon
+    type [f]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abcf↩     .
+    .de        .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 2  # cursor row
+    4 <- 0  # cursor column
+  ]
+]
+
+# if newline, move cursor to start of next line, and maybe align indent with previous line
+
+container editor-data [
+  indent?:boolean
+]
+
+after <editor-initialization> [
+  indent?:address:boolean <- get-address *result, indent?:offset
+  *indent? <- copy 1/true
+]
+
+scenario editor-moves-cursor-down-after-inserting-newline [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  assume-console [
+    type [0
+1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .0         .
+    .1abc      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-special-character> [
+  {
+    newline?:boolean <- equal *c, 10/newline
+    break-unless newline?
+    <insert-enter-begin>
+    editor <- insert-new-line-and-indent editor, screen
+    <insert-enter-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+recipe insert-new-line-and-indent [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen:address <- next-ingredient
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  screen-height:number <- screen-height screen
+  # insert newline
+  insert-duplex 10/newline, *before-cursor
+  *before-cursor <- next-duplex *before-cursor
+  *cursor-row <- add *cursor-row, 1
+  *cursor-column <- copy left
+  # maybe scroll
+  {
+    below-screen?:boolean <- greater-or-equal *cursor-row, screen-height  # must be equal, never greater
+    break-unless below-screen?
+    <scroll-down>
+    *cursor-row <- subtract *cursor-row, 1  # bring back into screen range
+  }
+  # indent if necessary
+  indent?:boolean <- get *editor, indent?:offset
+  reply-unless indent?, editor/same-as-ingredient:0, screen/same-as-ingredient:1
+  d:address:duplex-list <- get *editor, data:offset
+  end-of-previous-line:address:duplex-list <- prev-duplex *before-cursor
+  indent:number <- line-indent end-of-previous-line, d
+  i:number <- copy 0
+  {
+    indent-done?:boolean <- greater-or-equal i, indent
+    break-if indent-done?
+    editor, screen, go-render?:boolean <- insert-at-cursor editor, 32/space, screen
+    i <- add i, 1
+    loop
+  }
+  reply editor/same-as-ingredient:0, screen/same-as-ingredient:1
+]
+
+# takes a pointer 'curr' into the doubly-linked list and its sentinel, counts
+# the number of spaces at the start of the line containing 'curr'.
+recipe line-indent [
+  local-scope
+  curr:address:duplex-list <- next-ingredient
+  start:address:duplex-list <- next-ingredient
+  result:number <- copy 0
+  reply-unless curr, result
+  at-start?:boolean <- equal curr, start
+  reply-if at-start?, result
+  {
+    curr <- prev-duplex curr
+    break-unless curr
+    at-start?:boolean <- equal curr, start
+    break-if at-start?
+    c:character <- get *curr, value:offset
+    at-newline?:boolean <- equal c, 10/newline
+    break-if at-newline?
+    # if c is a space, increment result
+    is-space?:boolean <- equal c, 32/space
+    {
+      break-unless is-space?
+      result <- add result, 1
+    }
+    # if c is not a space, reset result
+    {
+      break-if is-space?
+      result <- copy 0
+    }
+    loop
+  }
+  reply result
+]
+
+scenario editor-moves-cursor-down-after-inserting-newline-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 1/left, 10/right
+  assume-console [
+    type [0
+1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    . 0        .
+    . 1abc     .
+    . ┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-clears-previous-line-completely-after-inserting-newline [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcde]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  assume-console [
+    press enter
+  ]
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .e         .
+    .          .
+    .          .
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # line should be fully cleared
+  screen-should-contain [
+    .          .
+    .          .
+    .abcd↩     .
+    .e         .
+    .┈┈┈┈┈     .
+  ]
+]
+
+scenario editor-inserts-indent-after-newline [
+  assume-screen 10/width, 10/height
+  1:address:array:character <- new [ab
+  cd
+ef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # position cursor after 'cd' and hit 'newline'
+  assume-console [
+    left-click 2, 8
+    type [
+]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor should be below start of previous line
+  memory-should-contain [
+    3 <- 3  # cursor row
+    4 <- 2  # cursor column (indented)
+  ]
+]
+
+scenario editor-skips-indent-around-paste [
+  assume-screen 10/width, 10/height
+  1:address:array:character <- new [ab
+  cd
+ef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # position cursor after 'cd' and hit 'newline' surrounded by paste markers
+  assume-console [
+    left-click 2, 8
+    press 65507  # start paste
+    press enter
+    press 65506  # end paste
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor should be below start of previous line
+  memory-should-contain [
+    3 <- 3  # cursor row
+    4 <- 0  # cursor column (not indented)
+  ]
+]
+
+after <handle-special-key> [
+  {
+    paste-start?:boolean <- equal *k, 65507/paste-start
+    break-unless paste-start?
+    indent?:address:boolean <- get-address *editor, indent?:offset
+    *indent? <- copy 0/false
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+after <handle-special-key> [
+  {
+    paste-end?:boolean <- equal *k, 65506/paste-end
+    break-unless paste-end?
+    indent?:address:boolean <- get-address *editor, indent?:offset
+    *indent? <- copy 1/true
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+## helpers
+
+recipe draw-horizontal [
+  local-scope
+  screen:address <- next-ingredient
+  row:number <- next-ingredient
+  x:number <- next-ingredient
+  right:number <- next-ingredient
+  style:character, style-found?:boolean <- next-ingredient
+  {
+    break-if style-found?
+    style <- copy 9472/horizontal
+  }
+  color:number, color-found?:boolean <- next-ingredient
+  {
+    # default color to white
+    break-if color-found?
+    color <- copy 245/grey
+  }
+  bg-color:number, bg-color-found?:boolean <- next-ingredient
+  {
+    break-if bg-color-found?
+    bg-color <- copy 0/black
+  }
+  screen <- move-cursor screen, row, x
+  {
+    continue?:boolean <- lesser-or-equal x, right  # right is inclusive, to match editor-data semantics
+    break-unless continue?
+    print-character screen, style, color, bg-color
+    x <- add x, 1
+    loop
+  }
+]
diff --git a/edit/003-shortcuts.mu b/edit/003-shortcuts.mu
new file mode 100644
index 00000000..879e2826
--- /dev/null
+++ b/edit/003-shortcuts.mu
@@ -0,0 +1,3069 @@
+## special shortcuts for manipulating the editor
+# Some keys on the keyboard generate unicode characters, others generate
+# terminfo key codes. We need to modify different places in the two cases.
+
+# tab - insert two spaces
+
+scenario editor-inserts-two-spaces-on-tab [
+  assume-screen 10/width, 5/height
+  # just one character in final line
+  1:address:array:character <- new [ab
+cd]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  assume-console [
+    press tab
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .  ab      .
+    .cd        .
+  ]
+]
+
+after <handle-special-character> [
+  {
+    tab?:boolean <- equal *c, 9/tab
+    break-unless tab?
+    <insert-character-begin>
+    editor, screen, go-render?:boolean <- insert-at-cursor editor, 32/space, screen
+    editor, screen, go-render?:boolean <- insert-at-cursor editor, 32/space, screen
+    <insert-character-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+# backspace - delete character before cursor
+
+scenario editor-handles-backspace-key [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 1
+    press backspace
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .bc        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    4 <- 1
+    5 <- 0
+  ]
+  check-trace-count-for-label 3, [print-character]  # length of original line to overwrite
+]
+
+after <handle-special-character> [
+  {
+    delete-previous-character?:boolean <- equal *c, 8/backspace
+    break-unless delete-previous-character?
+    <backspace-character-begin>
+    editor, screen, go-render?:boolean, backspaced-cell:address:duplex-list <- delete-before-cursor editor, screen
+    <backspace-character-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+# editor, screen, go-render?:boolean, backspaced-cell:address:duplex-list <- delete-before-cursor editor:address:editor-data, screen
+# return values:
+#   go-render? - whether caller needs to update the screen
+#   backspaced-cell - value deleted (or 0 if nothing was deleted) so we can save it for undo, etc.
+recipe delete-before-cursor [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen:address <- next-ingredient
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  # if at start of text (before-cursor at § sentinel), return
+  prev:address:duplex-list <- prev-duplex *before-cursor
+  reply-unless prev, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 0/no-more-render, 0/nothing-deleted
+  trace 10, [app], [delete-before-cursor]
+  original-row:number <- get *editor, cursor-row:offset
+  editor, scroll?:boolean <- move-cursor-coordinates-left editor
+  backspaced-cell:address:duplex-list <- copy *before-cursor
+  remove-duplex *before-cursor  # will also neatly trim next/prev pointers in backspaced-cell/*before-cursor
+  *before-cursor <- copy prev
+  reply-if scroll?, editor/same-as-ingredient:0, 1/go-render, backspaced-cell
+  screen-width:number <- screen-width screen
+  cursor-row:number <- get *editor, cursor-row:offset
+  cursor-column:number <- get *editor, cursor-column:offset
+  # did we just backspace over a newline?
+  same-row?:boolean <- equal cursor-row, original-row
+  reply-unless same-row?, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 1/go-render, backspaced-cell
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  curr:address:duplex-list <- next-duplex *before-cursor
+  screen <- move-cursor screen, cursor-row, cursor-column
+  curr-column:number <- copy cursor-column
+  {
+    # hit right margin? give up and let caller render
+    at-right?:boolean <- greater-or-equal curr-column, screen-width
+    reply-if at-right?, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 1/go-render, backspaced-cell
+    break-unless curr
+    # newline? done.
+    currc:character <- get *curr, value:offset
+    at-newline?:boolean <- equal currc, 10/newline
+    break-if at-newline?
+    screen <- print-character screen, currc
+    curr-column <- add curr-column, 1
+    curr <- next-duplex curr
+    loop
+  }
+  # we're guaranteed not to be at the right margin
+  screen <- print-character screen, 32/space
+  reply editor/same-as-ingredient:0, screen/same-as-ingredient:1, 0/no-more-render, backspaced-cell
+]
+
+recipe move-cursor-coordinates-left [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  before-cursor:address:duplex-list <- get *editor, before-cursor:offset
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  left:number <- get *editor, left:offset
+  # if not at left margin, move one character left
+  {
+    at-left-margin?:boolean <- equal *cursor-column, left
+    break-if at-left-margin?
+    trace 10, [app], [decrementing cursor column]
+    *cursor-column <- subtract *cursor-column, 1
+    reply editor/same-as-ingredient:0, 0/no-more-render
+  }
+  # if at left margin, we must move to previous row:
+  top-of-screen?:boolean <- equal *cursor-row, 1  # exclude menu bar
+  go-render?:boolean <- copy 0/false
+  {
+    break-if top-of-screen?
+    *cursor-row <- subtract *cursor-row, 1
+  }
+  {
+    break-unless top-of-screen?
+    <scroll-up>
+    go-render? <- copy 1/true
+  }
+  {
+    # case 1: if previous character was newline, figure out how long the previous line is
+    previous-character:character <- get *before-cursor, value:offset
+    previous-character-is-newline?:boolean <- equal previous-character, 10/newline
+    break-unless previous-character-is-newline?
+    # compute length of previous line
+    trace 10, [app], [switching to previous line]
+    d:address:duplex-list <- get *editor, data:offset
+    end-of-line:number <- previous-line-length before-cursor, d
+    *cursor-column <- add left, end-of-line
+    reply editor/same-as-ingredient:0, go-render?
+  }
+  # case 2: if previous-character was not newline, we're just at a wrapped line
+  trace 10, [app], [wrapping to previous line]
+  right:number <- get *editor, right:offset
+  *cursor-column <- subtract right, 1  # leave room for wrap icon
+  reply editor/same-as-ingredient:0, go-render?
+]
+
+# takes a pointer 'curr' into the doubly-linked list and its sentinel, counts
+# the length of the previous line before the 'curr' pointer.
+recipe previous-line-length [
+  local-scope
+  curr:address:duplex-list <- next-ingredient
+  start:address:duplex-list <- next-ingredient
+  result:number <- copy 0
+  reply-unless curr, result
+  at-start?:boolean <- equal curr, start
+  reply-if at-start?, result
+  {
+    curr <- prev-duplex curr
+    break-unless curr
+    at-start?:boolean <- equal curr, start
+    break-if at-start?
+    c:character <- get *curr, value:offset
+    at-newline?:boolean <- equal c, 10/newline
+    break-if at-newline?
+    result <- add result, 1
+    loop
+  }
+  reply result
+]
+
+scenario editor-clears-last-line-on-backspace [
+  assume-screen 10/width, 5/height
+  # just one character in final line
+  1:address:array:character <- new [ab
+cd]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  assume-console [
+    left-click 2, 0  # cursor at only character in final line
+    press backspace
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abcd      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    4 <- 1
+    5 <- 2
+  ]
+]
+
+# delete - delete character at cursor
+
+scenario editor-handles-delete-key [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    press delete
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .bc        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 3, [print-character]  # length of original line to overwrite
+  $clear-trace
+  assume-console [
+    press delete
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .c         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 2, [print-character]  # new length to overwrite
+]
+
+after <handle-special-key> [
+  {
+    delete-next-character?:boolean <- equal *k, 65522/delete
+    break-unless delete-next-character?
+    <delete-character-begin>
+    editor, screen, go-render?:boolean, deleted-cell:address:duplex-list <- delete-at-cursor editor, screen
+    <delete-character-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+recipe delete-at-cursor [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen:address <- next-ingredient
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  candidate:address:duplex-list <- next-duplex *before-cursor
+  reply-unless candidate, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 0/no-more-render, 0/nothing-deleted
+  currc:character <- get *candidate, value:offset
+  remove-duplex candidate
+  deleted-newline?:boolean <- equal currc, 10/newline
+  reply-if deleted-newline?, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 1/go-render, candidate/deleted-cell
+  # wasn't a newline? render rest of line
+  curr:address:duplex-list <- next-duplex *before-cursor  # refresh after remove-duplex above
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  screen <- move-cursor screen, *cursor-row, *cursor-column
+  curr-column:number <- copy *cursor-column
+  screen-width:number <- screen-width screen
+  {
+    # hit right margin? give up and let caller render
+    at-right?:boolean <- greater-or-equal curr-column, screen-width
+    reply-if at-right?, editor/same-as-ingredient:0, screen/same-as-ingredient:1, 1/go-render, candidate/deleted-cell
+    break-unless curr
+    # newline? done.
+    currc:character <- get *curr, value:offset
+    at-newline?:boolean <- equal currc, 10/newline
+    break-if at-newline?
+    screen <- print-character screen, currc
+    curr-column <- add curr-column, 1
+    curr <- next-duplex curr
+    loop
+  }
+  # we're guaranteed not to be at the right margin
+  screen <- print-character screen, 32/space
+  reply editor/same-as-ingredient:0, screen/same-as-ingredient:1, 0/no-more-render, candidate/deleted-cell
+]
+
+# right arrow
+
+scenario editor-moves-cursor-right-with-key [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    press right-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .a0bc      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 3, [print-character]  # 0 and following characters
+]
+
+after <handle-special-key> [
+  {
+    move-to-next-character?:boolean <- equal *k, 65514/right-arrow
+    break-unless move-to-next-character?
+    # if not at end of text
+    next-cursor:address:duplex-list <- next-duplex *before-cursor
+    break-unless next-cursor
+    # scan to next character
+    <move-cursor-begin>
+    *before-cursor <- copy next-cursor
+    editor, go-render?:boolean <- move-cursor-coordinates-right editor, screen-height
+    screen <- move-cursor screen, *cursor-row, *cursor-column
+    undo-coalesce-tag:number <- copy 2/right-arrow
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+recipe move-cursor-coordinates-right [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen-height:number <- next-ingredient
+  before-cursor:address:duplex-list <- get *editor before-cursor:offset
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  # if crossed a newline, move cursor to start of next row
+  {
+    old-cursor-character:character <- get *before-cursor, value:offset
+    was-at-newline?:boolean <- equal old-cursor-character, 10/newline
+    break-unless was-at-newline?
+    *cursor-row <- add *cursor-row, 1
+    *cursor-column <- copy left
+    below-screen?:boolean <- greater-or-equal *cursor-row, screen-height  # must be equal
+    reply-unless below-screen?, editor/same-as-ingredient:0, 0/no-more-render
+    <scroll-down>
+    *cursor-row <- subtract *cursor-row, 1  # bring back into screen range
+    reply editor/same-as-ingredient:0, 1/go-render
+  }
+  # if the line wraps, move cursor to start of next row
+  {
+    # if we're at the column just before the wrap indicator
+    wrap-column:number <- subtract right, 1
+    at-wrap?:boolean <- equal *cursor-column, wrap-column
+    break-unless at-wrap?
+    # and if next character isn't newline
+    next:address:duplex-list <- next-duplex before-cursor
+    break-unless next
+    next-character:character <- get *next, value:offset
+    newline?:boolean <- equal next-character, 10/newline
+    break-if newline?
+    *cursor-row <- add *cursor-row, 1
+    *cursor-column <- copy left
+    below-screen?:boolean <- greater-or-equal *cursor-row, screen-height  # must be equal
+    reply-unless below-screen?, editor/same-as-ingredient:0, 0/no-more-render
+    <scroll-down>
+    *cursor-row <- subtract *cursor-row, 1  # bring back into screen range
+    reply editor/same-as-ingredient:0, 1/go-render
+  }
+  # otherwise move cursor one character right
+  *cursor-column <- add *cursor-column, 1
+  reply editor/same-as-ingredient:0, 0/no-more-render
+]
+
+scenario editor-moves-cursor-to-next-line-with-right-arrow [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # type right-arrow a few times to get to start of second line
+  assume-console [
+    press right-arrow
+    press right-arrow
+    press right-arrow
+    press right-arrow  # next line
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  check-trace-count-for-label 0, [print-character]
+  # type something and ensure it goes where it should
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .0d        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 2, [print-character]  # new length of second line
+]
+
+scenario editor-moves-cursor-to-next-line-with-right-arrow-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 1/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    press right-arrow
+    press right-arrow
+    press right-arrow
+    press right-arrow  # next line
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    . abc      .
+    . 0d       .
+    . ┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-moves-cursor-to-next-wrapped-line-with-right-arrow [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcdef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 3
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .ef        .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-cursor-to-next-wrapped-line-with-right-arrow-2 [
+  assume-screen 10/width, 5/height
+  # line just barely wrapping
+  1:address:array:character <- new [abcde]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # position cursor at last character before wrap and hit right-arrow
+  assume-console [
+    left-click 1, 3
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 0
+  ]
+  # now hit right arrow again
+  assume-console [
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-cursor-to-next-wrapped-line-with-right-arrow-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abcdef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 1/left, 6/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 4
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .          .
+    . abcd↩    .
+    . ef       .
+    . ┈┈┈┈┈    .
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-cursor-to-next-line-with-right-arrow-at-end-of-line [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # move to end of line, press right-arrow, type a character
+  assume-console [
+    left-click 1, 3
+    press right-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # new character should be in next line
+  screen-should-contain [
+    .          .
+    .abc       .
+    .0d        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 2, [print-character]
+]
+
+# todo: ctrl-right: next word-end
+
+# left arrow
+
+scenario editor-moves-cursor-left-with-key [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 2
+    press left-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .a0bc      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 3, [print-character]
+]
+
+after <handle-special-key> [
+  {
+    move-to-previous-character?:boolean <- equal *k, 65515/left-arrow
+    break-unless move-to-previous-character?
+    trace 10, [app], [left arrow]
+    # if not at start of text (before-cursor at § sentinel)
+    prev:address:duplex-list <- prev-duplex *before-cursor
+    reply-unless prev, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    <move-cursor-begin>
+    editor, go-render? <- move-cursor-coordinates-left editor
+    *before-cursor <- copy prev
+    undo-coalesce-tag:number <- copy 1/left-arrow
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+scenario editor-moves-cursor-to-previous-line-with-left-arrow-at-start-of-line [
+  assume-screen 10/width, 5/height
+  # initialize editor with two lines
+  1:address:array:character <- new [abc
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # position cursor at start of second line (so there's no previous newline)
+  assume-console [
+    left-click 2, 0
+    press left-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-cursor-to-previous-line-with-left-arrow-at-start-of-line-2 [
+  assume-screen 10/width, 5/height
+  # initialize editor with three lines
+  1:address:array:character <- new [abc
+def
+g]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # position cursor further down (so there's a newline before the character at
+  # the cursor)
+  assume-console [
+    left-click 3, 0
+    press left-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def0      .
+    .g         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  check-trace-count-for-label 1, [print-character]  # just the '0'
+]
+
+scenario editor-moves-cursor-to-previous-line-with-left-arrow-at-start-of-line-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+g]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # position cursor at start of text, press left-arrow, then type a character
+  assume-console [
+    left-click 1, 0
+    press left-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # left-arrow should have had no effect
+  screen-should-contain [
+    .          .
+    .0abc      .
+    .def       .
+    .g         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  check-trace-count-for-label 4, [print-character]  # length of first line
+]
+
+scenario editor-moves-cursor-to-previous-line-with-left-arrow-at-start-of-line-4 [
+  assume-screen 10/width, 5/height
+  # initialize editor with text containing an empty line
+  1:address:array:character <- new [abc
+
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # position cursor right after empty line
+  assume-console [
+    left-click 3, 0
+    press left-arrow
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .0         .
+    .d         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  check-trace-count-for-label 1, [print-character]  # just the '0'
+]
+
+scenario editor-moves-across-screen-lines-across-wrap-with-left-arrow [
+  assume-screen 10/width, 5/height
+  # initialize editor with text containing an empty line
+  1:address:array:character <- new [abcdef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .ef        .
+    .┈┈┈┈┈     .
+    .          .
+  ]
+  # position cursor right after empty line
+  assume-console [
+    left-click 2, 0
+    press left-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1  # previous row
+    4 <- 3  # end of wrapped line
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+# todo: ctrl-left: previous word-start
+
+# up arrow
+
+scenario editor-moves-to-previous-line-with-up-arrow [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 2, 1
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .a0bc      .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-special-key> [
+  {
+    move-to-previous-line?:boolean <- equal *k, 65517/up-arrow
+    break-unless move-to-previous-line?
+    <move-cursor-begin>
+    editor, go-render? <- move-to-previous-line editor
+    undo-coalesce-tag:number <- copy 3/up-arrow
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+recipe move-to-previous-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  already-at-top?:boolean <- lesser-or-equal *cursor-row, 1/top
+  {
+    # if cursor not at top, move it
+    break-if already-at-top?
+    # if not at newline, move to start of line (previous newline)
+    # then scan back another line
+    # if either step fails, give up without modifying cursor or coordinates
+    curr:address:duplex-list <- copy *before-cursor
+    {
+      old:address:duplex-list <- copy curr
+      c2:character <- get *curr, value:offset
+      at-newline?:boolean <- equal c2, 10/newline
+      break-if at-newline?
+      curr:address:duplex-list <- before-previous-line curr, editor
+      no-motion?:boolean <- equal curr, old
+      reply-if no-motion?, editor/same-as-ingredient:0, 0/no-more-render
+    }
+    {
+      old <- copy curr
+      curr <- before-previous-line curr, editor
+      no-motion?:boolean <- equal curr, old
+      reply-if no-motion?, editor/same-as-ingredient:0, 0/no-more-render
+    }
+    *before-cursor <- copy curr
+    *cursor-row <- subtract *cursor-row, 1
+    # scan ahead to right column or until end of line
+    target-column:number <- copy *cursor-column
+    *cursor-column <- copy left
+    {
+      done?:boolean <- greater-or-equal *cursor-column, target-column
+      break-if done?
+      curr:address:duplex-list <- next-duplex *before-cursor
+      break-unless curr
+      currc:character <- get *curr, value:offset
+      at-newline?:boolean <- equal currc, 10/newline
+      break-if at-newline?
+      #
+      *before-cursor <- copy curr
+      *cursor-column <- add *cursor-column, 1
+      loop
+    }
+    reply editor/same-as-ingredient:0, 0/no-more-render
+  }
+  {
+    # if cursor already at top, scroll up
+    break-unless already-at-top?
+    <scroll-up>
+    reply editor/same-as-ingredient:0, 1/go-render
+  }
+]
+
+scenario editor-adjusts-column-at-previous-line [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [ab
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 2, 3
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .ab0       .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-adjusts-column-at-empty-line [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 2, 3
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .0         .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-moves-to-previous-line-from-left-margin [
+  assume-screen 10/width, 5/height
+  # start out with three lines
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # click on the third line and hit up-arrow, so you end up just after a newline
+  assume-console [
+    left-click 3, 0
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .0def      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+# down arrow
+
+scenario editor-moves-to-next-line-with-down-arrow [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # cursor starts out at (1, 0)
+  assume-console [
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # ..and ends at (2, 0)
+  memory-should-contain [
+    3 <- 2
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .0def      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-special-key> [
+  {
+    move-to-next-line?:boolean <- equal *k, 65516/down-arrow
+    break-unless move-to-next-line?
+    <move-cursor-begin>
+    editor, go-render? <- move-to-next-line editor, screen-height
+    undo-coalesce-tag:number <- copy 4/down-arrow
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, go-render?
+  }
+]
+
+recipe move-to-next-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen-height:number <- next-ingredient
+  cursor-row:address:number <- get-address *editor, cursor-row:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  last-line:number <- subtract screen-height, 1
+  already-at-bottom?:boolean <- greater-or-equal *cursor-row, last-line
+  {
+    # if cursor not at bottom, move it
+    break-if already-at-bottom?
+    # scan to start of next line, then to right column or until end of line
+    max:number <- subtract right, left
+    next-line:address:duplex-list <- before-start-of-next-line *before-cursor, max
+    {
+      # already at end of buffer? try to scroll up (so we can see more
+      # warnings or sandboxes below)
+      no-motion?:boolean <- equal next-line, *before-cursor
+      break-unless no-motion?
+      scroll?:boolean <- greater-than *cursor-row, 1
+      break-if scroll?, +try-to-scroll:label
+      reply editor/same-as-ingredient:0, 0/no-more-render
+    }
+    *cursor-row <- add *cursor-row, 1
+    *before-cursor <- copy next-line
+    target-column:number <- copy *cursor-column
+    *cursor-column <- copy left
+    {
+      done?:boolean <- greater-or-equal *cursor-column, target-column
+      break-if done?
+      curr:address:duplex-list <- next-duplex *before-cursor
+      break-unless curr
+      currc:character <- get *curr, value:offset
+      at-newline?:boolean <- equal currc, 10/newline
+      break-if at-newline?
+      #
+      *before-cursor <- copy curr
+      *cursor-column <- add *cursor-column, 1
+      loop
+    }
+    reply editor/same-as-ingredient:0, 0/no-more-render
+  }
+  +try-to-scroll
+  <scroll-down>
+  reply editor/same-as-ingredient:0, 1/go-render
+]
+
+scenario editor-adjusts-column-at-next-line [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+de]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  assume-console [
+    left-click 1, 3
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    3 <- 2
+    4 <- 2
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .de0       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-scrolls-at-end-on-down-arrow [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+de]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # try to move down past end of text
+  assume-console [
+    left-click 2, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen should scroll, moving cursor to end of text
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  assume-console [
+    type [0]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .de0       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # try to move down again
+  $clear-trace
+  assume-console [
+    left-click 2, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen stops scrolling because cursor is already at top
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .de01      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# ctrl-a/home - move cursor to start of line
+
+scenario editor-moves-to-start-of-line-with-ctrl-a [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on second line, press ctrl-a
+  assume-console [
+    left-click 2, 3
+    press ctrl-a
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to start of line
+  memory-should-contain [
+    4 <- 2
+    5 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+after <handle-special-character> [
+  {
+    move-to-start-of-line?:boolean <- equal *c, 1/ctrl-a
+    break-unless move-to-start-of-line?
+    <move-cursor-begin>
+    move-to-start-of-line editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+  }
+]
+
+after <handle-special-key> [
+  {
+    move-to-start-of-line?:boolean <- equal *k, 65521/home
+    break-unless move-to-start-of-line?
+    <move-cursor-begin>
+    move-to-start-of-line editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+  }
+]
+
+recipe move-to-start-of-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  # update cursor column
+  left:number <- get *editor, left:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  *cursor-column <- copy left
+  # update before-cursor
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  init:address:duplex-list <- get *editor, data:offset
+  # while not at start of line, move 
+  {
+    at-start-of-text?:boolean <- equal *before-cursor, init
+    break-if at-start-of-text?
+    prev:character <- get **before-cursor, value:offset
+    at-start-of-line?:boolean <- equal prev, 10/newline
+    break-if at-start-of-line?
+    *before-cursor <- prev-duplex *before-cursor
+    assert *before-cursor, [move-to-start-of-line tried to move before start of text]
+    loop
+  }
+]
+
+scenario editor-moves-to-start-of-line-with-ctrl-a-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on first line (no newline before), press ctrl-a
+  assume-console [
+    left-click 1, 3
+    press ctrl-a
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to start of line
+  memory-should-contain [
+    4 <- 1
+    5 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-to-start-of-line-with-home [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  $clear-trace
+  # start on second line, press 'home'
+  assume-console [
+    left-click 2, 3
+    press home
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to start of line
+  memory-should-contain [
+    3 <- 2
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-to-start-of-line-with-home-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on first line (no newline before), press 'home'
+  assume-console [
+    left-click 1, 3
+    press home
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to start of line
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+# ctrl-e/end - move cursor to end of line
+
+scenario editor-moves-to-end-of-line-with-ctrl-e [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on first line, press ctrl-e
+  assume-console [
+    left-click 1, 1
+    press ctrl-e
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to end of line
+  memory-should-contain [
+    4 <- 1
+    5 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+  # editor inserts future characters at cursor
+  assume-console [
+    type [z]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    4 <- 1
+    5 <- 4
+  ]
+  screen-should-contain [
+    .          .
+    .123z      .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  check-trace-count-for-label 1, [print-character]
+]
+
+after <handle-special-character> [
+  {
+    move-to-end-of-line?:boolean <- equal *c, 5/ctrl-e
+    break-unless move-to-end-of-line?
+    <move-cursor-begin>
+    move-to-end-of-line editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+  }
+]
+
+after <handle-special-key> [
+  {
+    move-to-end-of-line?:boolean <- equal *k, 65520/end
+    break-unless move-to-end-of-line?
+    <move-cursor-begin>
+    move-to-end-of-line editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+  }
+]
+
+recipe move-to-end-of-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  # while not at start of line, move 
+  {
+    next:address:duplex-list <- next-duplex *before-cursor
+    break-unless next  # end of text
+    nextc:character <- get *next, value:offset
+    at-end-of-line?:boolean <- equal nextc, 10/newline
+    break-if at-end-of-line?
+    *before-cursor <- copy next
+    *cursor-column <- add *cursor-column, 1
+    loop
+  }
+]
+
+scenario editor-moves-to-end-of-line-with-ctrl-e-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on second line (no newline after), press ctrl-e
+  assume-console [
+    left-click 2, 1
+    press ctrl-e
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    4:number <- get *2:address:editor-data, cursor-row:offset
+    5:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to end of line
+  memory-should-contain [
+    4 <- 2
+    5 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-to-end-of-line-with-end [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on first line, press 'end'
+  assume-console [
+    left-click 1, 1
+    press end
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to end of line
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+scenario editor-moves-to-end-of-line-with-end-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  $clear-trace
+  # start on second line (no newline after), press 'end'
+  assume-console [
+    left-click 2, 1
+    press end
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to end of line
+  memory-should-contain [
+    3 <- 2
+    4 <- 3
+  ]
+  check-trace-count-for-label 0, [print-character]
+]
+
+# ctrl-u - delete text from start of line until (but not at) cursor
+
+scenario editor-deletes-to-start-of-line-with-ctrl-u [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start on second line, press ctrl-u
+  assume-console [
+    left-click 2, 2
+    press ctrl-u
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to start of line
+  screen-should-contain [
+    .          .
+    .123       .
+    .6         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-special-character> [
+  {
+    delete-to-start-of-line?:boolean <- equal *c, 21/ctrl-u
+    break-unless delete-to-start-of-line?
+    <delete-to-start-of-line-begin>
+    deleted-cells:address:duplex-list <- delete-to-start-of-line editor
+    <delete-to-start-of-line-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+recipe delete-to-start-of-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  # compute range to delete
+  init:address:duplex-list <- get *editor, data:offset
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  start:address:duplex-list <- copy *before-cursor
+  end:address:duplex-list <- next-duplex *before-cursor
+  {
+    at-start-of-text?:boolean <- equal start, init
+    break-if at-start-of-text?
+    curr:character <- get *start, value:offset
+    at-start-of-line?:boolean <- equal curr, 10/newline
+    break-if at-start-of-line?
+    start <- prev-duplex start
+    assert start, [delete-to-start-of-line tried to move before start of text]
+    loop
+  }
+  # snip it out
+  result:address:duplex-list <- next-duplex start
+  remove-duplex-between start, end
+  # adjust cursor
+  *before-cursor <- prev-duplex end
+  left:number <- get *editor, left:offset
+  cursor-column:address:number <- get-address *editor, cursor-column:offset
+  *cursor-column <- copy left
+  reply result
+]
+
+scenario editor-deletes-to-start-of-line-with-ctrl-u-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start on first line (no newline before), press ctrl-u
+  assume-console [
+    left-click 1, 2
+    press ctrl-u
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to start of line
+  screen-should-contain [
+    .          .
+    .3         .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-start-of-line-with-ctrl-u-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start past end of line, press ctrl-u
+  assume-console [
+    left-click 1, 3
+    press ctrl-u
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to start of line
+  screen-should-contain [
+    .          .
+    .          .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-start-of-final-line-with-ctrl-u [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start past end of final line, press ctrl-u
+  assume-console [
+    left-click 2, 3
+    press ctrl-u
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to start of line
+  screen-should-contain [
+    .          .
+    .123       .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# ctrl-k - delete text from cursor to end of line (but not the newline)
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start on first line, press ctrl-k
+  assume-console [
+    left-click 1, 1
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to end of line
+  screen-should-contain [
+    .          .
+    .1         .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-special-character> [
+  {
+    delete-to-end-of-line?:boolean <- equal *c, 11/ctrl-k
+    break-unless delete-to-end-of-line?
+    <delete-to-end-of-line-begin>
+    deleted-cells:address:duplex-list <- delete-to-end-of-line editor
+    <delete-to-end-of-line-end>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+recipe delete-to-end-of-line [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  # compute range to delete
+  start:address:duplex-list <- get *editor, before-cursor:offset
+  end:address:duplex-list <- next-duplex start
+  {
+    at-end-of-text?:boolean <- equal end, 0/null
+    break-if at-end-of-text?
+    curr:character <- get *end, value:offset
+    at-end-of-line?:boolean <- equal curr, 10/newline
+    break-if at-end-of-line?
+    end <- next-duplex end
+    loop
+  }
+  # snip it out
+  result:address:duplex-list <- next-duplex start
+  remove-duplex-between start, end
+  reply result
+]
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k-2 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start on second line (no newline after), press ctrl-k
+  assume-console [
+    left-click 2, 1
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes to end of line
+  screen-should-contain [
+    .          .
+    .123       .
+    .4         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k-3 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start at end of line
+  assume-console [
+    left-click 1, 2
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes just last character
+  screen-should-contain [
+    .          .
+    .12        .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k-4 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start past end of line
+  assume-console [
+    left-click 1, 3
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes nothing
+  screen-should-contain [
+    .          .
+    .123       .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k-5 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start at end of text
+  assume-console [
+    left-click 2, 2
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes just the final character
+  screen-should-contain [
+    .          .
+    .123       .
+    .45        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-deletes-to-end-of-line-with-ctrl-k-6 [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [123
+456]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  # start past end of text
+  assume-console [
+    left-click 2, 3
+    press ctrl-k
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # cursor deletes nothing
+  screen-should-contain [
+    .          .
+    .123       .
+    .456       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# cursor-down can scroll if necessary
+
+scenario editor-can-scroll-down-using-arrow-keys [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with >3 lines
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # position cursor at last line, then try to move further down
+  assume-console [
+    left-click 3, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen slides by one line
+  screen-should-contain [
+    .          .
+    .b         .
+    .c         .
+    .d         .
+  ]
+]
+
+after <scroll-down> [
+  trace 10, [app], [scroll down]
+  top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  max:number <- subtract right, left
+  old-top:address:duplex-list <- copy *top-of-screen
+  *top-of-screen <- before-start-of-next-line *top-of-screen, max
+  no-movement?:boolean <- equal old-top, *top-of-screen
+  # Hack: this reply doesn't match one of the locations of <scroll-down>,
+  # directly within insert-at-cursor. However, I'm unable to trigger the
+  # error.. If necessary create a duplicate copy of <scroll-down> with the
+  # right 'reply-if'.
+  reply-if no-movement?, editor/same-as-ingredient:0, 0/no-more-render
+]
+
+# takes a pointer into the doubly-linked list, scans ahead at most 'max'
+# positions until the next newline
+# beware: never return null pointer.
+recipe before-start-of-next-line [
+  local-scope
+  original:address:duplex-list <- next-ingredient
+  max:number <- next-ingredient
+  count:number <- copy 0
+  curr:address:duplex-list <- copy original
+  # skip the initial newline if it exists
+  {
+    c:character <- get *curr, value:offset
+    at-newline?:boolean <- equal c, 10/newline
+    break-unless at-newline?
+    curr <- next-duplex curr
+    count <- add count, 1
+  }
+  {
+    reply-unless curr, original
+    done?:boolean <- greater-or-equal count, max
+    break-if done?
+    c:character <- get *curr, value:offset
+    at-newline?:boolean <- equal c, 10/newline
+    break-if at-newline?
+    curr <- next-duplex curr
+    count <- add count, 1
+    loop
+  }
+  reply-unless curr, original
+  reply curr
+]
+
+scenario editor-scrolls-down-past-wrapped-line-using-arrow-keys [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with a long, wrapped line and more than a screen of
+  # other lines
+  1:address:array:character <- new [abcdef
+g
+h
+i]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .ef        .
+    .g         .
+  ]
+  # position cursor at last line, then try to move further down
+  assume-console [
+    left-click 3, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .ef        .
+    .g         .
+    .h         .
+  ]
+]
+
+scenario editor-scrolls-down-past-wrapped-line-using-arrow-keys-2 [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # editor starts with a long line wrapping twice
+  1:address:array:character <- new [abcdefghij
+k
+l
+m]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at last line, then try to move further down
+  assume-console [
+    left-click 3, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line containing a wrap icon
+  screen-should-contain [
+    .          .
+    .efgh↩     .
+    .ij        .
+    .k         .
+  ]
+  # scroll down again
+  assume-console [
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .ij        .
+    .k         .
+    .l         .
+  ]
+]
+
+scenario editor-scrolls-down-when-line-wraps [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 5/width, 4/height
+  # editor contains a long line in the third line
+  1:address:array:character <- new [a
+b
+cdef]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at end, type a character
+  assume-console [
+    left-click 3, 4
+    type [g]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .cdef↩.
+    .g    .
+  ]
+  memory-should-contain [
+    3 <- 3
+    4 <- 1
+  ]
+]
+
+scenario editor-scrolls-down-on-newline [
+  assume-screen 5/width, 4/height
+  # position cursor after last line and type newline
+  1:address:array:character <- new [a
+b
+c]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  assume-console [
+    left-click 3, 4
+    type [
+]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .c    .
+    .     .
+  ]
+  memory-should-contain [
+    3 <- 3
+    4 <- 0
+  ]
+]
+
+scenario editor-scrolls-down-on-right-arrow [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 5/width, 4/height
+  # editor contains a wrapped line
+  1:address:array:character <- new [a
+b
+cdefgh]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at end of screen and try to move right
+  assume-console [
+    left-click 3, 3
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .cdef↩.
+    .gh   .
+  ]
+  memory-should-contain [
+    3 <- 3
+    4 <- 0
+  ]
+]
+
+scenario editor-scrolls-down-on-right-arrow-2 [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 5/width, 4/height
+  # editor contains more lines than can fit on screen
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at end of screen and try to move right
+  assume-console [
+    left-click 3, 3
+    press right-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .c    .
+    .d    .
+  ]
+  memory-should-contain [
+    3 <- 3
+    4 <- 0
+  ]
+]
+
+scenario editor-combines-page-and-line-scroll [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with a few pages of lines
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f
+g]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # scroll down one page and one line
+  assume-console [
+    press page-down
+    left-click 3, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen scrolls down 3 lines
+  screen-should-contain [
+    .          .
+    .d         .
+    .e         .
+    .f         .
+  ]
+]
+
+# cursor-up can scroll if necessary
+
+scenario editor-can-scroll-up-using-arrow-keys [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with >3 lines
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # position cursor at top of second page, then try to move up
+  assume-console [
+    press page-down
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen slides by one line
+  screen-should-contain [
+    .          .
+    .b         .
+    .c         .
+    .d         .
+  ]
+]
+
+after <scroll-up> [
+  trace 10, [app], [scroll up]
+  top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+  old-top:address:duplex-list <- copy *top-of-screen
+  *top-of-screen <- before-previous-line *top-of-screen, editor
+  no-movement?:boolean <- equal old-top, *top-of-screen
+  reply-if no-movement?, editor/same-as-ingredient:0, 0/no-more-render
+]
+
+# takes a pointer into the doubly-linked list, scans back to before start of
+# previous *wrapped* line
+# beware: never return null pointer
+recipe before-previous-line [
+  local-scope
+  curr:address:duplex-list <- next-ingredient
+  c:character <- get *curr, value:offset
+  # compute max, number of characters to skip
+  #   1 + len%(width-1)
+  #   except rotate second term to vary from 1 to width-1 rather than 0 to width-2
+  editor:address:editor-data <- next-ingredient
+  left:number <- get *editor, left:offset
+  right:number <- get *editor, right:offset
+  max-line-length:number <- subtract right, left, -1/exclusive-right, 1/wrap-icon
+  sentinel:address:duplex-list <- get *editor, data:offset
+  len:number <- previous-line-length curr, sentinel
+  {
+    break-if len
+    # empty line; just skip this newline
+    prev:address:duplex-list <- prev-duplex curr
+    reply-unless prev, curr
+    reply prev
+  }
+  _, max:number <- divide-with-remainder len, max-line-length
+  # remainder 0 => scan one width-worth
+  {
+    break-if max
+    max <- copy max-line-length
+  }
+  max <- add max, 1
+  count:number <- copy 0
+  # skip 'max' characters
+  {
+    done?:boolean <- greater-or-equal count, max
+    break-if done?
+    prev:address:duplex-list <- prev-duplex curr
+    break-unless prev
+    curr <- copy prev
+    count <- add count, 1
+    loop
+  }
+  reply curr
+]
+
+scenario editor-scrolls-up-past-wrapped-line-using-arrow-keys [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with a long, wrapped line and more than a screen of
+  # other lines
+  1:address:array:character <- new [abcdef
+g
+h
+i]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .ef        .
+    .g         .
+  ]
+  # position cursor at top of second page, just below wrapped line
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .g         .
+    .h         .
+    .i         .
+  ]
+  # now move up one line
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .ef        .
+    .g         .
+    .h         .
+  ]
+]
+
+scenario editor-scrolls-up-past-wrapped-line-using-arrow-keys-2 [
+  # screen has 1 line for menu + 4 lines
+  assume-screen 10/width, 5/height
+  # editor starts with a long line wrapping twice, occupying 3 of the 4 lines
+  1:address:array:character <- new [abcdefghij
+k
+l
+m]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at top of second page
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .k         .
+    .l         .
+    .m         .
+    .┈┈┈┈┈     .
+  ]
+  # move up one line
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .ij        .
+    .k         .
+    .l         .
+    .m         .
+  ]
+  # move up a second line
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .efgh↩     .
+    .ij        .
+    .k         .
+    .l         .
+  ]
+  # move up a third line
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .abcd↩     .
+    .efgh↩     .
+    .ij        .
+    .k         .
+  ]
+]
+
+# same as editor-scrolls-up-past-wrapped-line-using-arrow-keys but length
+# slightly off, just to prevent over-training
+scenario editor-scrolls-up-past-wrapped-line-using-arrow-keys-3 [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with a long, wrapped line and more than a screen of
+  # other lines
+  1:address:array:character <- new [abcdef
+g
+h
+i]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 6/right
+  screen-should-contain [
+    .          .
+    .abcde↩    .
+    .f         .
+    .g         .
+  ]
+  # position cursor at top of second page, just below wrapped line
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .g         .
+    .h         .
+    .i         .
+  ]
+  # now move up one line
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows partial wrapped line
+  screen-should-contain [
+    .          .
+    .f         .
+    .g         .
+    .h         .
+  ]
+]
+
+# check empty lines
+scenario editor-scrolls-up-past-wrapped-line-using-arrow-keys-4 [
+  assume-screen 10/width, 4/height
+  # initialize editor with some lines around an empty line
+  1:address:array:character <- new [a
+b
+
+c
+d
+e]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 6/right
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .          .
+    .c         .
+    .d         .
+  ]
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .d         .
+    .e         .
+    .┈┈┈┈┈┈    .
+  ]
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .          .
+    .c         .
+    .d         .
+  ]
+]
+
+scenario editor-scrolls-up-on-left-arrow [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 5/width, 4/height
+  # editor contains >3 lines
+  1:address:array:character <- new [a
+b
+c
+d
+e]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at top of second page
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .     .
+    .c    .
+    .d    .
+    .e    .
+  ]
+  # now try to move left
+  assume-console [
+    press left-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .c    .
+    .d    .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+]
+
+scenario editor-can-scroll-up-to-start-of-file [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with >3 lines
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # position cursor at top of second page, then try to move up to start of
+  # text
+  assume-console [
+    press page-down
+    press up-arrow
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen slides by one line
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # try to move up again
+  assume-console [
+    press up-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen remains unchanged
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+]
+
+# ctrl-f/page-down - render next page if it exists
+
+scenario editor-can-scroll [
+  assume-screen 10/width, 4/height
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows next page
+  screen-should-contain [
+    .          .
+    .c         .
+    .d         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+after <handle-special-character> [
+  {
+    page-down?:boolean <- equal *c, 6/ctrl-f
+    break-unless page-down?
+    top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+    old-top:address:duplex-list <- copy *top-of-screen
+    <move-cursor-begin>
+    page-down editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    no-movement?:boolean <- equal *top-of-screen, old-top
+    reply-if no-movement?, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+after <handle-special-key> [
+  {
+    page-down?:boolean <- equal *k, 65518/page-down
+    break-unless page-down?
+    top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+    old-top:address:duplex-list <- copy *top-of-screen
+    <move-cursor-begin>
+    page-down editor
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    no-movement?:boolean <- equal *top-of-screen, old-top
+    reply-if no-movement?, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+# page-down skips entire wrapped lines, so it can't scroll past lines
+# taking up the entire screen
+recipe page-down [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  # if editor contents don't overflow screen, do nothing
+  bottom-of-screen:address:duplex-list <- get *editor, bottom-of-screen:offset
+  reply-unless bottom-of-screen, editor/same-as-ingredient:0
+  # if not, position cursor at final character
+  before-cursor:address:address:duplex-list <- get-address *editor, before-cursor:offset
+  *before-cursor <- prev-duplex bottom-of-screen
+  # keep one line in common with previous page
+  {
+    last:character <- get **before-cursor, value:offset
+    newline?:boolean <- equal last, 10/newline
+    break-unless newline?:boolean
+    *before-cursor <- prev-duplex *before-cursor
+  }
+  # move cursor and top-of-screen to start of that line
+  move-to-start-of-line editor
+  top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+  *top-of-screen <- copy *before-cursor
+  reply editor/same-as-ingredient:0
+]
+
+scenario editor-does-not-scroll-past-end [
+  assume-screen 10/width, 4/height
+  1:address:array:character <- new [a
+b]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen remains unmodified
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-starts-next-page-at-start-of-wrapped-line [
+  # screen has 1 line for menu + 3 lines for text
+  assume-screen 10/width, 4/height
+  # editor contains a long last line
+  1:address:array:character <- new [a
+b
+cdefgh]
+  # editor screen triggers wrap of last line
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  # some part of last line is not displayed
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .cde↩      .
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows entire wrapped line
+  screen-should-contain [
+    .          .
+    .cde↩      .
+    .fgh       .
+    .┈┈┈┈      .
+  ]
+]
+
+scenario editor-starts-next-page-at-start-of-wrapped-line-2 [
+  # screen has 1 line for menu + 3 lines for text
+  assume-screen 10/width, 4/height
+  # editor contains a very long line that occupies last two lines of screen
+  # and still has something left over
+  1:address:array:character <- new [a
+bcdefgh]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  # some part of last line is not displayed
+  screen-should-contain [
+    .          .
+    .a         .
+    .bcd↩      .
+    .efg↩      .
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows entire wrapped line
+  screen-should-contain [
+    .          .
+    .bcd↩      .
+    .efg↩      .
+    .h         .
+  ]
+]
+
+# ctrl-b/page-up - render previous page if it exists
+
+scenario editor-can-scroll-up [
+  assume-screen 10/width, 4/height
+  1:address:array:character <- new [a
+b
+c
+d]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows next page
+  screen-should-contain [
+    .          .
+    .c         .
+    .d         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  # scroll back up
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows original page again
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+]
+
+after <handle-special-character> [
+  {
+    page-up?:boolean <- equal *c, 2/ctrl-b
+    break-unless page-up?
+    top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+    old-top:address:duplex-list <- copy *top-of-screen
+    <move-cursor-begin>
+    editor <- page-up editor, screen-height
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    no-movement?:boolean <- equal *top-of-screen, old-top
+    reply-if no-movement?, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+after <handle-special-key> [
+  {
+    page-up?:boolean <- equal *k, 65519/page-up
+    break-unless page-up?
+    top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+    old-top:address:duplex-list <- copy *top-of-screen
+    <move-cursor-begin>
+    editor <- page-up editor, screen-height
+    undo-coalesce-tag:number <- copy 0/never
+    <move-cursor-end>
+    no-movement?:boolean <- equal *top-of-screen, old-top
+    # don't bother re-rendering if nothing changed. todo: test this
+    reply-if no-movement?, screen/same-as-ingredient:0, editor/same-as-ingredient:1, 0/no-more-render
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+recipe page-up [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  screen-height:number <- next-ingredient
+  max:number <- subtract screen-height, 1/menu-bar, 1/overlapping-line
+  count:number <- copy 0
+  top-of-screen:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+  {
+    done?:boolean <- greater-or-equal count, max
+    break-if done?
+    prev:address:duplex-list <- before-previous-line *top-of-screen, editor
+    break-unless prev
+    *top-of-screen <- copy prev
+    count <- add count, 1
+    loop
+  }
+  reply editor/same-as-ingredient:0
+]
+
+scenario editor-can-scroll-up-multiple-pages [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 10/width, 4/height
+  # initialize editor with 8 lines
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f
+g
+h]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+  # scroll down two pages
+  assume-console [
+    press page-down
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows third page
+  screen-should-contain [
+    .          .
+    .e         .
+    .f         .
+    .g         .
+  ]
+  # scroll up
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows second page
+  screen-should-contain [
+    .          .
+    .c         .
+    .d         .
+    .e         .
+  ]
+  # scroll up again
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows original page again
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+  ]
+]
+
+scenario editor-can-scroll-up-wrapped-lines [
+  # screen has 1 line for menu + 5 lines for text
+  assume-screen 10/width, 6/height
+  # editor contains a long line in the first page
+  1:address:array:character <- new [a
+b
+cdefgh
+i
+j
+k
+l
+m
+n
+o]
+  # editor screen triggers wrap of last line
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  # some part of last line is not displayed
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .cde↩      .
+    .fgh       .
+    .i         .
+  ]
+  # scroll down a page and a line
+  assume-console [
+    press page-down
+    left-click 5, 0
+    press down-arrow
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows entire wrapped line
+  screen-should-contain [
+    .          .
+    .j         .
+    .k         .
+    .l         .
+    .m         .
+    .n         .
+  ]
+  # now scroll up one page
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen resets
+  screen-should-contain [
+    .          .
+    .b         .
+    .cde↩      .
+    .fgh       .
+    .i         .
+    .j         .
+  ]
+]
+
+scenario editor-can-scroll-up-wrapped-lines-2 [
+  # screen has 1 line for menu + 3 lines for text
+  assume-screen 10/width, 4/height
+  # editor contains a very long line that occupies last two lines of screen
+  # and still has something left over
+  1:address:array:character <- new [a
+bcdefgh]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  # some part of last line is not displayed
+  screen-should-contain [
+    .          .
+    .a         .
+    .bcd↩      .
+    .efg↩      .
+  ]
+  # scroll down
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen shows entire wrapped line
+  screen-should-contain [
+    .          .
+    .bcd↩      .
+    .efg↩      .
+    .h         .
+  ]
+  # scroll back up
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # screen resets
+  screen-should-contain [
+    .          .
+    .a         .
+    .bcd↩      .
+    .efg↩      .
+  ]
+]
+
+scenario editor-can-scroll-up-past-nonempty-lines [
+  assume-screen 10/width, 4/height
+  # text with empty line in second screen
+  1:address:array:character <- new [axx
+bxx
+cxx
+dxx
+exx
+fxx
+gxx
+hxx
+]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  screen-should-contain [
+    .          .
+    .axx       .
+    .bxx       .
+    .cxx       .
+  ]
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .cxx       .
+    .dxx       .
+    .exx       .
+  ]
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .exx       .
+    .fxx       .
+    .gxx       .
+  ]
+  # scroll back up past empty line
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .cxx       .
+    .dxx       .
+    .exx       .
+  ]
+]
+
+scenario editor-can-scroll-up-past-empty-lines [
+  assume-screen 10/width, 4/height
+  # text with empty line in second screen
+  1:address:array:character <- new [axy
+bxy
+cxy
+
+dxy
+exy
+fxy
+gxy
+]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 4/right
+  screen-should-contain [
+    .          .
+    .axy       .
+    .bxy       .
+    .cxy       .
+  ]
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .cxy       .
+    .          .
+    .dxy       .
+  ]
+  assume-console [
+    press page-down
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .dxy       .
+    .exy       .
+    .fxy       .
+  ]
+  # scroll back up past empty line
+  assume-console [
+    press page-up
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .cxy       .
+    .          .
+    .dxy       .
+  ]
+]
diff --git a/edit/004-programming-environment.mu b/edit/004-programming-environment.mu
new file mode 100644
index 00000000..9698572f
--- /dev/null
+++ b/edit/004-programming-environment.mu
@@ -0,0 +1,652 @@
+## putting the environment together out of editors
+#
+# Consists of one editor on the left for recipes and one on the right for the
+# sandbox.
+
+recipe main [
+  local-scope
+  open-console
+  initial-recipe:address:array:character <- restore [recipes.mu]
+  initial-sandbox:address:array:character <- new []
+  hide-screen 0/screen
+  env:address:programming-environment-data <- new-programming-environment 0/screen, initial-recipe, initial-sandbox
+  env <- restore-sandboxes env
+  render-all 0/screen, env
+  event-loop 0/screen, 0/console, env
+  # never gets here
+]
+
+container programming-environment-data [
+  recipes:address:editor-data
+  recipe-warnings:address:array:character
+  current-sandbox:address:editor-data
+  sandbox:address:sandbox-data  # list of sandboxes, from top to bottom
+  sandbox-in-focus?:boolean  # false => cursor in recipes; true => cursor in current-sandbox
+]
+
+recipe new-programming-environment [
+  local-scope
+  screen:address <- next-ingredient
+  initial-recipe-contents:address:array:character <- next-ingredient
+  initial-sandbox-contents:address:array:character <- next-ingredient
+  width:number <- screen-width screen
+  height:number <- screen-height screen
+  # top menu
+  result:address:programming-environment-data <- new programming-environment-data:type
+  draw-horizontal screen, 0, 0/left, width, 32/space, 0/black, 238/grey
+  button-start:number <- subtract width, 20
+  button-on-screen?:boolean <- greater-or-equal button-start, 0
+  assert button-on-screen?, [screen too narrow for menu]
+  screen <- move-cursor screen, 0/row, button-start
+  run-button:address:array:character <- new [ run (F4) ]
+  print-string screen, run-button, 255/white, 161/reddish
+  # dotted line down the middle
+  divider:number, _ <- divide-with-remainder width, 2
+  draw-vertical screen, divider, 1/top, height, 9482/vertical-dotted
+  # recipe editor on the left
+  recipes:address:address:editor-data <- get-address *result, recipes:offset
+  *recipes <- new-editor initial-recipe-contents, screen, 0/left, divider/right
+  # sandbox editor on the right
+  new-left:number <- add divider, 1
+  current-sandbox:address:address:editor-data <- get-address *result, current-sandbox:offset
+  *current-sandbox <- new-editor initial-sandbox-contents, screen, new-left, width/right
+  +programming-environment-initialization
+  reply result
+]
+
+recipe event-loop [
+  local-scope
+  screen:address <- next-ingredient
+  console:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  recipes:address:editor-data <- get *env, recipes:offset
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  sandbox-in-focus?:address:boolean <- get-address *env, sandbox-in-focus?:offset
+  # if we fall behind we'll stop updating the screen, but then we have to
+  # render the entire screen when we catch up.
+  # todo: test this
+  render-all-on-no-more-events?:boolean <- copy 0/false
+  {
+    # looping over each (keyboard or touch) event as it occurs
+    +next-event
+    e:event, console, found?:boolean, quit?:boolean <- read-event console
+    loop-unless found?
+    break-if quit?  # only in tests
+    trace 10, [app], [next-event]
+    <handle-event>
+    # check for global events that will trigger regardless of which editor has focus
+    {
+      k:address:number <- maybe-convert e:event, keycode:variant
+      break-unless k
+      <global-keypress>
+    }
+    {
+      c:address:character <- maybe-convert e:event, text:variant
+      break-unless c
+      <global-type>
+    }
+    # 'touch' event - send to both sides, see what picks it up
+    {
+      t:address:touch-event <- maybe-convert e:event, touch:variant
+      break-unless t
+      # ignore all but 'left-click' events for now
+      # todo: test this
+      touch-type:number <- get *t, type:offset
+      is-left-click?:boolean <- equal touch-type, 65513/mouse-left
+      loop-unless is-left-click?, +next-event:label
+      # later exceptions for non-editor touches will go here
+      <global-touch>
+      # send to both editors
+      _ <- move-cursor-in-editor screen, recipes, *t
+      *sandbox-in-focus? <- move-cursor-in-editor screen, current-sandbox, *t
+      screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+      loop +next-event:label
+    }
+    # 'resize' event - redraw editor
+    # todo: test this after supporting resize in assume-console
+    {
+      r:address:resize-event <- maybe-convert e:event, resize:variant
+      break-unless r
+      # if more events, we're still resizing; wait until we stop
+      more-events?:boolean <- has-more-events? console
+      {
+        break-unless more-events?
+        render-all-on-no-more-events? <- copy 1/true  # no rendering now, full rendering on some future event
+      }
+      {
+        break-if more-events?
+        env <- resize screen, env
+        screen <- render-all screen, env
+        render-all-on-no-more-events? <- copy 0/false  # full render done
+      }
+      loop +next-event:label
+    }
+    # if it's not global and not a touch event, send to appropriate editor
+    {
+      hide-screen screen
+      {
+        break-if *sandbox-in-focus?
+        screen, recipes, render?:boolean <- handle-keyboard-event screen, recipes, e:event
+        # refresh screen only if no more events
+        # if there are more events to process, wait for them to clear up, then make sure you render-all afterward.
+        more-events?:boolean <- has-more-events? console
+        {
+          break-unless more-events?
+          render-all-on-no-more-events? <- copy 1/true  # no rendering now, full rendering on some future event
+          jump +finish-event:label
+        }
+        {
+          break-if more-events?
+          {
+            break-unless render-all-on-no-more-events?
+            # no more events, and we have to force render
+            screen <- render-all screen, env
+            render-all-on-no-more-events? <- copy 0/false
+            jump +finish-event:label
+          }
+          # no more events, no force render
+          {
+            break-unless render?
+            screen <- render-recipes screen, env
+            jump +finish-event:label
+          }
+        }
+      }
+      {
+        break-unless *sandbox-in-focus?
+        screen, current-sandbox, render?:boolean <- handle-keyboard-event screen, current-sandbox, e:event
+        # refresh screen only if no more events
+        # if there are more events to process, wait for them to clear up, then make sure you render-all afterward.
+        more-events?:boolean <- has-more-events? console
+        {
+          break-unless more-events?
+          render-all-on-no-more-events? <- copy 1/true  # no rendering now, full rendering on some future event
+          jump +finish-event:label
+        }
+        {
+          break-if more-events?
+          {
+            break-unless render-all-on-no-more-events?
+            # no more events, and we have to force render
+            screen <- render-all screen, env
+            render-all-on-no-more-events? <- copy 0/false
+            jump +finish-event:label
+          }
+          # no more events, no force render
+          {
+            break-unless render?
+            screen <- render-sandbox-side screen, env
+            jump +finish-event:label
+          }
+        }
+      }
+      +finish-event
+      screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+      show-screen screen
+    }
+    loop
+  }
+]
+
+recipe resize [
+  local-scope
+  screen:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  clear-screen screen  # update screen dimensions
+  width:number <- screen-width screen
+  divider:number, _ <- divide-with-remainder width, 2
+  # update recipe editor
+  recipes:address:editor-data <- get *env, recipes:offset
+  right:address:number <- get-address *recipes, right:offset
+  *right <- subtract divider, 1
+  # reset cursor (later we'll try to preserve its position)
+  cursor-row:address:number <- get-address *recipes, cursor-row:offset
+  *cursor-row <- copy 1
+  cursor-column:address:number <- get-address *recipes, cursor-column:offset
+  *cursor-column <- copy 0
+  # update sandbox editor
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  left:address:number <- get-address *current-sandbox, left:offset
+  right:address:number <- get-address *current-sandbox, right:offset
+  *left <- add divider, 1
+  *right <- subtract width, 1
+  # reset cursor (later we'll try to preserve its position)
+  cursor-row:address:number <- get-address *current-sandbox, cursor-row:offset
+  *cursor-row <- copy 1
+  cursor-column:address:number <- get-address *current-sandbox, cursor-column:offset
+  *cursor-column <- copy *left
+  reply env/same-as-ingredient:1
+]
+
+scenario point-at-multiple-editors [
+  $close-trace  # trace too long
+  assume-screen 30/width, 5/height
+  # initialize both halves of screen
+  1:address:array:character <- new [abc]
+  2:address:array:character <- new [def]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # focus on both sides
+  assume-console [
+    left-click 1, 1
+    left-click 1, 17
+  ]
+  # check cursor column in each
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    4:address:editor-data <- get *3:address:programming-environment-data, recipes:offset
+    5:number <- get *4:address:editor-data, cursor-column:offset
+    6:address:editor-data <- get *3:address:programming-environment-data, current-sandbox:offset
+    7:number <- get *6:address:editor-data, cursor-column:offset
+  ]
+  memory-should-contain [
+    5 <- 1
+    7 <- 17
+  ]
+]
+
+scenario edit-multiple-editors [
+  $close-trace  # trace too long
+  assume-screen 30/width, 5/height
+  # initialize both halves of screen
+  1:address:array:character <- new [abc]
+  2:address:array:character <- new [def]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  render-all screen, 3:address:programming-environment-data
+  # type one letter in each of them
+  assume-console [
+    left-click 1, 1
+    type [0]
+    left-click 1, 17
+    type [1]
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    4:address:editor-data <- get *3:address:programming-environment-data, recipes:offset
+    5:number <- get *4:address:editor-data, cursor-column:offset
+    6:address:editor-data <- get *3:address:programming-environment-data, current-sandbox:offset
+    7:number <- get *6:address:editor-data, cursor-column:offset
+  ]
+  screen-should-contain [
+    .           run (F4)           .  # this line has a different background, but we don't test that yet
+    .a0bc           ┊d1ef          .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+  memory-should-contain [
+    5 <- 2  # cursor column of recipe editor
+    7 <- 18  # cursor column of sandbox editor
+  ]
+  # show the cursor at the right window
+  run [
+    print-character screen:address, 9251/␣/cursor
+  ]
+  screen-should-contain [
+    .           run (F4)           .
+    .a0bc           ┊d1␣f          .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+]
+
+scenario multiple-editors-cover-only-their-own-areas [
+  $close-trace  # trace too long
+  assume-screen 60/width, 10/height
+  run [
+    1:address:array:character <- new [abc]
+    2:address:array:character <- new [def]
+    3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+    render-all screen, 3:address:programming-environment-data
+  ]
+  # divider isn't messed up
+  screen-should-contain [
+    .                                         run (F4)           .
+    .abc                           ┊def                          .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                              ┊                             .
+    .                              ┊                             .
+  ]
+]
+
+scenario editor-in-focus-keeps-cursor [
+  $close-trace  # trace too long
+  assume-screen 30/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:array:character <- new [def]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  render-all screen, 3:address:programming-environment-data
+  # initialize programming environment and highlight cursor
+  assume-console []
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    print-character screen:address, 9251/␣/cursor
+  ]
+  # is cursor at the right place?
+  screen-should-contain [
+    .           run (F4)           .
+    .␣bc            ┊def           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+  # now try typing a letter
+  assume-console [
+    type [z]
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    print-character screen:address, 9251/␣/cursor
+  ]
+  # cursor should still be right
+  screen-should-contain [
+    .           run (F4)           .
+    .z␣bc           ┊def           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+]
+
+scenario backspace-in-sandbox-editor-joins-lines [
+  $close-trace  # trace too long
+  assume-screen 30/width, 5/height
+  # initialize sandbox side with two lines
+  1:address:array:character <- new []
+  2:address:array:character <- new [abc
+def]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  render-all screen, 3:address:programming-environment-data
+  screen-should-contain [
+    .           run (F4)           .
+    .               ┊abc           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊def           .
+    .               ┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+  # position cursor at start of second line and hit backspace
+  assume-console [
+    left-click 2, 16
+    press backspace
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    print-character screen:address, 9251/␣/cursor
+  ]
+  # cursor moves to end of old line
+  screen-should-contain [
+    .           run (F4)           .
+    .               ┊abc␣ef        .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+]
+
+recipe render-all [
+  local-scope
+  screen:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  trace 10, [app], [render all]
+  hide-screen screen
+  # top menu
+  trace 11, [app], [render top menu]
+  width:number <- screen-width screen
+  draw-horizontal screen, 0, 0/left, width, 32/space, 0/black, 238/grey
+  button-start:number <- subtract width, 20
+  button-on-screen?:boolean <- greater-or-equal button-start, 0
+  assert button-on-screen?, [screen too narrow for menu]
+  screen <- move-cursor screen, 0/row, button-start
+  run-button:address:array:character <- new [ run (F4) ]
+  print-string screen, run-button, 255/white, 161/reddish
+  # error message
+  trace 11, [app], [render status]
+  recipe-warnings:address:array:character <- get *env, recipe-warnings:offset
+  {
+    break-unless recipe-warnings
+    status:address:array:character <- new [errors found]
+    update-status screen, status, 1/red
+  }
+  # dotted line down the middle
+  trace 11, [app], [render divider]
+  divider:number, _ <- divide-with-remainder width, 2
+  height:number <- screen-height screen
+  draw-vertical screen, divider, 1/top, height, 9482/vertical-dotted
+  #
+  screen <- render-recipes screen, env
+  screen <- render-sandbox-side screen, env
+  #
+  recipes:address:editor-data <- get *env, recipes:offset
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  sandbox-in-focus?:boolean <- get *env, sandbox-in-focus?:offset
+  screen <- update-cursor screen, recipes, current-sandbox, sandbox-in-focus?
+  #
+  show-screen screen
+  reply screen/same-as-ingredient:0
+]
+
+recipe render-recipes [
+  local-scope
+  screen:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  trace 11, [app], [render recipes]
+  recipes:address:editor-data <- get *env, recipes:offset
+  # render recipes
+  left:number <- get *recipes, left:offset
+  right:number <- get *recipes, right:offset
+  row:number, column:number, screen <- render screen, recipes
+  clear-line-delimited screen, column, right
+  recipe-warnings:address:array:character <- get *env, recipe-warnings:offset
+  {
+    # print any warnings
+    break-unless recipe-warnings
+    row, screen <- render-string screen, recipe-warnings, left, right, 1/red, row
+  }
+  {
+    # no warnings? move to next line
+    break-if recipe-warnings
+    row <- add row, 1
+  }
+  # draw dotted line after recipes
+  draw-horizontal screen, row, left, right, 9480/horizontal-dotted
+  row <- add row, 1
+  clear-screen-from screen, row, left, left, right
+  reply screen/same-as-ingredient:0
+]
+
+recipe update-cursor [
+  local-scope
+  screen:address <- next-ingredient
+  recipes:address:editor-data <- next-ingredient
+  current-sandbox:address:editor-data <- next-ingredient
+  sandbox-in-focus?:boolean <- next-ingredient
+  {
+    break-if sandbox-in-focus?
+    cursor-row:number <- get *recipes, cursor-row:offset
+    cursor-column:number <- get *recipes, cursor-column:offset
+  }
+  {
+    break-unless sandbox-in-focus?
+    cursor-row:number <- get *current-sandbox, cursor-row:offset
+    cursor-column:number <- get *current-sandbox, cursor-column:offset
+  }
+  screen <- move-cursor screen, cursor-row, cursor-column
+  reply screen/same-as-ingredient:0
+]
+
+# ctrl-l - redraw screen (just in case it printed junk somehow)
+
+after <global-type> [
+  {
+    redraw-screen?:boolean <- equal *c, 12/ctrl-l
+    break-unless redraw-screen?
+    screen <- render-all screen, env:address:programming-environment-data
+    sync-screen screen
+    loop +next-event:label
+  }
+]
+
+# ctrl-n - switch focus
+# todo: test this
+
+after <global-type> [
+  {
+    switch-side?:boolean <- equal *c, 14/ctrl-n
+    break-unless switch-side?
+    *sandbox-in-focus? <- not *sandbox-in-focus?
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    loop +next-event:label
+  }
+]
+
+# ctrl-x - maximize/unmaximize the side with focus
+
+scenario maximize-side [
+  $close-trace  # trace too long
+  assume-screen 30/width, 5/height
+  # initialize both halves of screen
+  1:address:array:character <- new [abc]
+  2:address:array:character <- new [def]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  screen <- render-all screen, 3:address:programming-environment-data
+  screen-should-contain [
+    .           run (F4)           .
+    .abc            ┊def           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+  # hit ctrl-x
+  assume-console [
+    press ctrl-x
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # only left side visible
+  screen-should-contain [
+    .           run (F4)           .
+    .abc                           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈.
+    .                              .
+  ]
+  # hit any key to toggle back
+  assume-console [
+    press ctrl-x
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .           run (F4)           .
+    .abc            ┊def           .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━.
+    .               ┊              .
+  ]
+]
+
+#? # ctrl-t - browse trace
+#? after <global-type> [
+#?   {
+#?     browse-trace?:boolean <- equal *c, 20/ctrl-t
+#?     break-unless browse-trace?
+#?     $browse-trace
+#?     screen <- render-all screen, env:address:programming-environment-data
+#?     loop +next-event:label
+#?   }
+#? ]
+
+container programming-environment-data [
+  maximized?:boolean
+]
+
+after <global-type> [
+  {
+    maximize?:boolean <- equal *c, 24/ctrl-x
+    break-unless maximize?
+    screen, console <- maximize screen, console, env:address:programming-environment-data
+    loop +next-event:label
+  }
+]
+
+recipe maximize [
+  local-scope
+  screen:address <- next-ingredient
+  console:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  hide-screen screen
+  # maximize one of the sides
+  maximized?:address:boolean <- get-address *env, maximized?:offset
+  *maximized? <- copy 1/true
+  #
+  sandbox-in-focus?:boolean <- get *env, sandbox-in-focus?:offset
+  {
+    break-if sandbox-in-focus?
+    editor:address:editor-data <- get *env, recipes:offset
+    right:address:number <- get-address *editor, right:offset
+    *right <- screen-width screen
+    *right <- subtract *right, 1
+    screen <- render-recipes screen, env
+  }
+  {
+    break-unless sandbox-in-focus?
+    editor:address:editor-data <- get *env, current-sandbox:offset
+    left:address:number <- get-address *editor, left:offset
+    *left <- copy 0
+    screen <- render-sandbox-side screen, env
+  }
+  show-screen screen
+  reply screen/same-as-ingredient:0, console/same-as-ingredient:1
+]
+
+# when maximized, wait for any event and simply unmaximize
+after <handle-event> [
+  {
+    maximized?:address:boolean <- get-address *env, maximized?:offset
+    break-unless *maximized?
+    *maximized? <- copy 0/false
+    # undo maximize
+    {
+      break-if *sandbox-in-focus?
+      editor:address:editor-data <- get *env, recipes:offset
+      right:address:number <- get-address *editor, right:offset
+      *right <- screen-width screen
+      *right <- divide *right, 2
+      *right <- subtract *right, 1
+    }
+    {
+      break-unless *sandbox-in-focus?
+      editor:address:editor-data <- get *env, current-sandbox:offset
+      left:address:number <- get-address *editor, left:offset
+      *left <- screen-width screen
+      *left <- divide *left, 2
+      *left <- add *left, 1
+    }
+    render-all screen, env
+    show-screen screen
+    loop +next-event:label
+  }
+]
+
+## helpers
+
+recipe draw-vertical [
+  local-scope
+  screen:address <- next-ingredient
+  col:number <- next-ingredient
+  y:number <- next-ingredient
+  bottom:number <- next-ingredient
+  style:character, style-found?:boolean <- next-ingredient
+  {
+    break-if style-found?
+    style <- copy 9474/vertical
+  }
+  color:number, color-found?:boolean <- next-ingredient
+  {
+    # default color to white
+    break-if color-found?
+    color <- copy 245/grey
+  }
+  {
+    continue?:boolean <- lesser-than y, bottom
+    break-unless continue?
+    screen <- move-cursor screen, y, col
+    print-character screen, style, color
+    y <- add y, 1
+    loop
+  }
+]
diff --git a/edit/005-sandbox.mu b/edit/005-sandbox.mu
new file mode 100644
index 00000000..239112ce
--- /dev/null
+++ b/edit/005-sandbox.mu
@@ -0,0 +1,683 @@
+## running code from the editor and creating sandboxes
+
+container sandbox-data [
+  data:address:array:character
+  response:address:array:character
+  warnings:address:array:character
+  trace:address:array:character
+  expected-response:address:array:character
+  # coordinates to track clicks
+  starting-row-on-screen:number
+  code-ending-row-on-screen:number
+  response-starting-row-on-screen:number
+  display-trace?:boolean
+  screen:address:screen  # prints in the sandbox go here
+  next-sandbox:address:sandbox-data
+]
+
+scenario run-and-show-results [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  # recipe editor is empty
+  1:address:array:character <- new []
+  # sandbox editor contains an instruction without storing outputs
+  2:address:array:character <- new [divide-with-remainder 11, 3]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the code in the editors
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen prints the results
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊divide-with-remainder 11, 3                      .
+    .                                                  ┊3                                                .
+    .                                                  ┊2                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  screen-should-contain-in-color 7/white, [
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                   divide-with-remainder 11, 3                      .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+  ]
+  screen-should-contain-in-color 245/grey, [
+    .                                                                                                    .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊                                                 .
+    .                                                  ┊3                                                .
+    .                                                  ┊2                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  # run another command
+  assume-console [
+    left-click 1, 80
+    type [add 2, 2]
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen prints the results
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊add 2, 2                                         .
+    .                                                  ┊4                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊divide-with-remainder 11, 3                      .
+    .                                                  ┊3                                                .
+    .                                                  ┊2                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+# hook into event-loop recipe: read non-unicode keypress from k, process it if
+# necessary, then go to next level
+after <global-keypress> [
+  # F4? load all code and run all sandboxes.
+  {
+    do-run?:boolean <- equal *k, 65532/F4
+    break-unless do-run?
+    status:address:array:character <- new [running...  ]
+    screen <- update-status screen, status, 245/grey
+    screen, error?:boolean <- run-sandboxes env, screen
+    # F4 might update warnings and results on both sides
+    screen <- render-all screen, env
+    {
+      break-if error?
+      status:address:array:character <- new [            ]
+      screen <- update-status screen, status, 245/grey
+    }
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    loop +next-event:label
+  }
+]
+
+recipe run-sandboxes [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  screen:address <- next-ingredient
+  recipes:address:editor-data <- get *env, recipes:offset
+  # copy code from recipe editor, persist, load into mu, save any warnings
+  in:address:array:character <- editor-contents recipes
+  save [recipes.mu], in
+  recipe-warnings:address:address:array:character <- get-address *env, recipe-warnings:offset
+  *recipe-warnings <- reload in
+  # if recipe editor has errors, stop
+  {
+    break-unless *recipe-warnings
+    status:address:array:character <- new [errors found]
+    update-status screen, status, 1/red
+    reply screen/same-as-ingredient:1, 1/errors-found
+  }
+  # check contents of right editor (sandbox)
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  {
+    sandbox-contents:address:array:character <- editor-contents current-sandbox
+    break-unless sandbox-contents
+    # if contents exist, first save them
+    # run them and turn them into a new sandbox-data
+    new-sandbox:address:sandbox-data <- new sandbox-data:type
+    data:address:address:array:character <- get-address *new-sandbox, data:offset
+    *data <- copy sandbox-contents
+    # push to head of sandbox list
+    dest:address:address:sandbox-data <- get-address *env, sandbox:offset
+    next:address:address:sandbox-data <- get-address *new-sandbox, next-sandbox:offset
+    *next <- copy *dest
+    *dest <- copy new-sandbox
+    # clear sandbox editor
+    init:address:address:duplex-list <- get-address *current-sandbox, data:offset
+    *init <- push-duplex 167/§, 0/tail
+    top-of-screen:address:address:duplex-list <- get-address *current-sandbox, top-of-screen:offset
+    *top-of-screen <- copy *init
+  }
+  # save all sandboxes before running, just in case we die when running
+  save-sandboxes env
+  # run all sandboxes
+  curr:address:sandbox-data <- get *env, sandbox:offset
+  {
+    break-unless curr
+    data <- get-address *curr, data:offset
+    response:address:address:array:character <- get-address *curr, response:offset
+    warnings:address:address:array:character <- get-address *curr, warnings:offset
+    trace:address:address:array:character <- get-address *curr, trace:offset
+    fake-screen:address:address:screen <- get-address *curr, screen:offset
+    *response, *warnings, *fake-screen, *trace, completed?:boolean <- run-interactive *data
+    {
+      break-if *warnings
+      break-if completed?:boolean
+      *warnings <- new [took too long!
+]
+    }
+    curr <- get *curr, next-sandbox:offset
+    loop
+  }
+  reply screen/same-as-ingredient:1, 0/no-errors-found
+]
+
+recipe update-status [
+  local-scope
+  screen:address <- next-ingredient
+  msg:address:array:character <- next-ingredient
+  color:number <- next-ingredient
+  screen <- move-cursor screen, 0, 2
+  screen <- print-string screen, msg, color, 238/grey/background
+  reply screen/same-as-ingredient:0
+]
+
+recipe save-sandboxes [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  # first clear previous versions, in case we deleted some sandbox
+  $system [rm lesson/[0-9]* >/dev/null 2>/dev/null]  # some shells can't handle '>&'
+  curr:address:sandbox-data <- get *env, sandbox:offset
+  suffix:address:array:character <- new [.out]
+  idx:number <- copy 0
+  {
+    break-unless curr
+    data:address:array:character <- get *curr, data:offset
+    filename:address:array:character <- integer-to-decimal-string idx
+    save filename, data
+    {
+      expected-response:address:array:character <- get *curr, expected-response:offset
+      break-unless expected-response
+      filename <- string-append filename, suffix
+      save filename, expected-response
+    }
+    idx <- add idx, 1
+    curr <- get *curr, next-sandbox:offset
+    loop
+  }
+]
+
+recipe render-sandbox-side [
+  local-scope
+  screen:address <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  trace 11, [app], [render sandbox side]
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  left:number <- get *current-sandbox, left:offset
+  right:number <- get *current-sandbox, right:offset
+  row:number, column:number, screen, current-sandbox <- render screen, current-sandbox
+  clear-screen-from screen, row, column, left, right
+  row <- add row, 1
+  draw-horizontal screen, row, left, right, 9473/horizontal-double
+  sandbox:address:sandbox-data <- get *env, sandbox:offset
+  row, screen <- render-sandboxes screen, sandbox, left, right, row
+  clear-rest-of-screen screen, row, left, left, right
+  reply screen/same-as-ingredient:0
+]
+
+recipe render-sandboxes [
+  local-scope
+  screen:address <- next-ingredient
+  sandbox:address:sandbox-data <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  row:number <- next-ingredient
+  reply-unless sandbox, row/same-as-ingredient:4, screen/same-as-ingredient:0
+  screen-height:number <- screen-height screen
+  at-bottom?:boolean <- greater-or-equal row, screen-height
+  reply-if at-bottom?:boolean, row/same-as-ingredient:4, screen/same-as-ingredient:0
+  # render sandbox menu
+  row <- add row, 1
+  screen <- move-cursor screen, row, left
+  clear-line-delimited screen, left, right
+  print-character screen, 120/x, 245/grey
+  # save menu row so we can detect clicks to it later
+  starting-row:address:number <- get-address *sandbox, starting-row-on-screen:offset
+  *starting-row <- copy row
+  # render sandbox contents
+  sandbox-data:address:array:character <- get *sandbox, data:offset
+  row, screen <- render-code-string screen, sandbox-data, left, right, row
+  code-ending-row:address:number <- get-address *sandbox, code-ending-row-on-screen:offset
+  *code-ending-row <- copy row
+  # render sandbox warnings, screen or response, in that order
+  response-starting-row:address:number <- get-address *sandbox, response-starting-row-on-screen:offset
+  sandbox-response:address:array:character <- get *sandbox, response:offset
+  sandbox-warnings:address:array:character <- get *sandbox, warnings:offset
+  sandbox-screen:address <- get *sandbox, screen:offset
+  <render-sandbox-results>
+  {
+    break-unless sandbox-warnings
+    *response-starting-row <- copy 0  # no response
+    row, screen <- render-string screen, sandbox-warnings, left, right, 1/red, row
+  }
+  {
+    break-if sandbox-warnings
+    empty-screen?:boolean <- fake-screen-is-empty? sandbox-screen
+    break-if empty-screen?
+    row, screen <- render-screen screen, sandbox-screen, left, right, row
+  }
+  {
+    break-if sandbox-warnings
+    break-unless empty-screen?
+    *response-starting-row <- add row, 1
+    <render-sandbox-response>
+    row, screen <- render-string screen, sandbox-response, left, right, 245/grey, row
+  }
+  +render-sandbox-end
+  at-bottom?:boolean <- greater-or-equal row, screen-height
+  reply-if at-bottom?, row/same-as-ingredient:4, screen/same-as-ingredient:0
+  # draw solid line after sandbox
+  draw-horizontal screen, row, left, right, 9473/horizontal-double
+  # draw next sandbox
+  next-sandbox:address:sandbox-data <- get *sandbox, next-sandbox:offset
+  row, screen <- render-sandboxes screen, next-sandbox, left, right, row
+  reply row/same-as-ingredient:4, screen/same-as-ingredient:0
+]
+
+# assumes programming environment has no sandboxes; restores them from previous session
+recipe restore-sandboxes [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  # read all scenarios, pushing them to end of a list of scenarios
+  suffix:address:array:character <- new [.out]
+  idx:number <- copy 0
+  curr:address:address:sandbox-data <- get-address *env, sandbox:offset
+  {
+    filename:address:array:character <- integer-to-decimal-string idx
+    contents:address:array:character <- restore filename
+    break-unless contents  # stop at first error; assuming file didn't exist
+    # create new sandbox for file
+    *curr <- new sandbox-data:type
+    data:address:address:array:character <- get-address **curr, data:offset
+    *data <- copy contents
+    # restore expected output for sandbox if it exists
+    {
+      filename <- string-append filename, suffix
+      contents <- restore filename
+      break-unless contents
+      expected-response:address:address:array:character <- get-address **curr, expected-response:offset
+      *expected-response <- copy contents
+    }
+    +continue
+    idx <- add idx, 1
+    curr <- get-address **curr, next-sandbox:offset
+    loop
+  }
+  reply env/same-as-ingredient:0
+]
+
+# row, screen <- render-screen screen:address, sandbox-screen:address, left:number, right:number, row:number
+# print the fake sandbox screen to 'screen' with appropriate delimiters
+# leave cursor at start of next line
+recipe render-screen [
+  local-scope
+  screen:address <- next-ingredient
+  s:address:screen <- next-ingredient
+  left:number <- next-ingredient
+  right:number <- next-ingredient
+  row:number <- next-ingredient
+  row <- add row, 1
+  reply-unless s, row/same-as-ingredient:4, screen/same-as-ingredient:0
+  # print 'screen:'
+  header:address:array:character <- new [screen:]
+  row <- subtract row, 1  # compensate for render-string below
+  row <- render-string screen, header, left, right, 245/grey, row
+  # newline
+  row <- add row, 1
+  screen <- move-cursor screen, row, left
+  # start printing s
+  column:number <- copy left
+  s-width:number <- screen-width s
+  s-height:number <- screen-height s
+  buf:address:array:screen-cell <- get *s, data:offset
+  stop-printing:number <- add left, s-width, 3
+  max-column:number <- min stop-printing, right
+  i:number <- copy 0
+  len:number <- length *buf
+  screen-height:number <- screen-height screen
+  {
+    done?:boolean <- greater-or-equal i, len
+    break-if done?
+    done? <- greater-or-equal row, screen-height
+    break-if done?
+    column <- copy left
+    screen <- move-cursor screen, row, column
+    # initial leader for each row: two spaces and a '.'
+    print-character screen, 32/space, 245/grey
+    print-character screen, 32/space, 245/grey
+    print-character screen, 46/full-stop, 245/grey
+    column <- add left, 3
+    {
+      # print row
+      row-done?:boolean <- greater-or-equal column, max-column
+      break-if row-done?
+      curr:screen-cell <- index *buf, i
+      c:character <- get curr, contents:offset
+      color:number <- get curr, color:offset
+      {
+        # damp whites down to grey
+        white?:boolean <- equal color, 7/white
+        break-unless white?
+        color <- copy 245/grey
+      }
+      print-character screen, c, color
+      column <- add column, 1
+      i <- add i, 1
+      loop
+    }
+    # print final '.'
+    print-character screen, 46/full-stop, 245/grey
+    column <- add column, 1
+    {
+      # clear rest of current line
+      line-done?:boolean <- greater-than column, right
+      break-if line-done?
+      print-character screen, 32/space
+      column <- add column, 1
+      loop
+    }
+    row <- add row, 1
+    loop
+  }
+  reply row/same-as-ingredient:4, screen/same-as-ingredient:0
+]
+
+scenario run-updates-results [
+  $close-trace  # trace too long
+  assume-screen 100/width, 12/height
+  # define a recipe (no indent for the 'add' line below so column numbers are more obvious)
+  1:address:array:character <- new [ 
+recipe foo [
+z:number <- add 2, 2
+]]
+  # sandbox editor contains an instruction without storing outputs
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the code in the editors
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen prints the results
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .z:number <- add 2, 2                              ┊                                                x.
+    .]                                                 ┊foo                                              .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊4                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  # make a change (incrementing one of the args to 'add'), then rerun
+  assume-console [
+    left-click 3, 28  # one past the value of the second arg
+    press backspace
+    type [3]
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen updates the result on the right
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .z:number <- add 2, 3                              ┊                                                x.
+    .]                                                 ┊foo                                              .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊5                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-instruction-and-print-warnings [
+  $close-trace  # trace too long
+  assume-screen 100/width, 10/height
+  # left editor is empty
+  1:address:array:character <- new []
+  # right editor contains an illegal instruction
+  2:address:array:character <- new [get 1234:number, foo:offset]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the code in the editors
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen prints error message in red
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊get 1234:number, foo:offset                      .
+    .                                                  ┊unknown element foo in container number          .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  screen-should-contain-in-color 7/white, [
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                   get 1234:number, foo:offset                      .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+  ]
+  screen-should-contain-in-color 1/red, [
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                   unknown element foo in container number          .
+    .                                                                                                    .
+  ]
+  screen-should-contain-in-color 245/grey, [
+    .                                                                                                    .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊                                                 .
+    .                                                  ┊                                                 .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-instruction-and-print-warnings-only-once [
+  $close-trace  # trace too long
+  assume-screen 100/width, 10/height
+  # left editor is empty
+  1:address:array:character <- new []
+  # right editor contains an illegal instruction
+  2:address:array:character <- new [get 1234:number, foo:offset]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the code in the editors multiple times
+  assume-console [
+    press F4
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that screen prints error message just once
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊get 1234:number, foo:offset                      .
+    .                                                  ┊unknown element foo in container number          .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-instruction-manages-screen-per-sandbox [
+  $close-trace  # trace too long
+  assume-screen 100/width, 20/height
+  # left editor is empty
+  1:address:array:character <- new []
+  # right editor contains an instruction
+  2:address:array:character <- new [print-integer screen:address, 4]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the code in the editor
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # check that it prints a little toy screen
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊print-integer screen:address, 4                  .
+    .                                                  ┊screen:                                          .
+    .                                                  ┊  .4                             .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario sandbox-with-print-can-be-edited [
+  $close-trace  # trace too long
+  assume-screen 100/width, 20/height
+  # left editor is empty
+  1:address:array:character <- new []
+  # right editor contains an instruction
+  2:address:array:character <- new [print-integer screen:address, 4]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the sandbox
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊print-integer screen:address, 4                  .
+    .                                                  ┊screen:                                          .
+    .                                                  ┊  .4                             .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊  .                              .               .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  # edit the sandbox
+  assume-console [
+    left-click 3, 70
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊print-integer screen:address, 4                  .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario sandbox-can-handle-infinite-loop [
+  $close-trace  # trace too long
+  assume-screen 100/width, 20/height
+  # left editor is empty
+  1:address:array:character <- new [recipe foo [
+  {
+    loop
+  }
+]]
+  # right editor contains an instruction
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run the sandbox
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .recipe foo [                                      ┊                                                 .
+    .  {                                               ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .    loop                                          ┊                                                x.
+    .  }                                               ┊foo                                              .
+    .]                                                 ┊took too long!                                   .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+recipe editor-contents [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  buf:address:buffer <- new-buffer 80
+  curr:address:duplex-list <- get *editor, data:offset
+  # skip § sentinel
+  assert curr, [editor without data is illegal; must have at least a sentinel]
+  curr <- next-duplex curr
+  reply-unless curr, 0
+  {
+    break-unless curr
+    c:character <- get *curr, value:offset
+    buffer-append buf, c
+    curr <- next-duplex curr
+    loop
+  }
+  result:address:array:character <- buffer-to-array buf
+  reply result
+]
+
+scenario editor-provides-edited-contents [
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  assume-console [
+    left-click 1, 2
+    type [def]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:address:array:character <- editor-contents 2:address:editor-data
+    4:array:character <- copy *3:address:array:character
+  ]
+  memory-should-contain [
+    4:string <- [abdefc]
+  ]
+]
diff --git a/edit/006-sandbox-edit.mu b/edit/006-sandbox-edit.mu
new file mode 100644
index 00000000..acd597c7
--- /dev/null
+++ b/edit/006-sandbox-edit.mu
@@ -0,0 +1,128 @@
+## editing sandboxes after they've been created
+
+scenario clicking-on-a-sandbox-moves-it-to-editor [
+  $close-trace  # trace too long
+  assume-screen 40/width, 10/height
+  # basic recipe
+  1:address:array:character <- new [ 
+recipe foo [
+  add 2, 2
+]]
+  # run it
+  2:address:array:character <- new [foo]
+  assume-console [
+    press F4
+  ]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  event-loop screen:address, console:address, 3:address:programming-environment-data
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  add 2, 2          ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊4                  .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # click somewhere on the sandbox
+  assume-console [
+    left-click 3, 30
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # it pops back into editor
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊foo                .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  add 2, 2          ┊                   .
+    .]                   ┊                   .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                   .
+    .                    ┊                   .
+    .                    ┊                   .
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [0]
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊0foo               .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  add 2, 2          ┊                   .
+    .]                   ┊                   .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                   .
+    .                    ┊                   .
+    .                    ┊                   .
+  ]
+]
+
+after <global-touch> [
+  # right side of screen and below sandbox editor? pop appropriate sandbox
+  # contents back into sandbox editor provided it's empty
+  {
+    sandbox-left-margin:number <- get *current-sandbox, left:offset
+    click-column:number <- get *t, column:offset
+    on-sandbox-side?:boolean <- greater-or-equal click-column, sandbox-left-margin
+    break-unless on-sandbox-side?
+    first-sandbox:address:sandbox-data <- get *env, sandbox:offset
+    break-unless first-sandbox
+    first-sandbox-begins:number <- get *first-sandbox, starting-row-on-screen:offset
+    click-row:number <- get *t, row:offset
+    below-sandbox-editor?:boolean <- greater-or-equal click-row, first-sandbox-begins
+    break-unless below-sandbox-editor?
+    empty-sandbox-editor?:boolean <- empty-editor? current-sandbox
+    break-unless empty-sandbox-editor?  # make the user hit F4 before editing a new sandbox
+    # identify the sandbox to edit and remove it from the sandbox list
+    sandbox:address:sandbox-data <- extract-sandbox env, click-row
+    text:address:array:character <- get *sandbox, data:offset
+    current-sandbox <- insert-text current-sandbox, text
+    hide-screen screen
+    screen <- render-sandbox-side screen, env
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    show-screen screen
+    loop +next-event:label
+  }
+]
+
+recipe empty-editor? [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  head:address:duplex-list <- get *editor, data:offset
+  first:address:duplex-list <- next-duplex head
+  result:boolean <- not first
+  reply result
+]
+
+recipe extract-sandbox [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  click-row:number <- next-ingredient
+  # assert click-row >= sandbox.starting-row-on-screen
+  sandbox:address:address:sandbox-data <- get-address *env, sandbox:offset
+  start:number <- get **sandbox, starting-row-on-screen:offset
+  clicked-on-sandboxes?:boolean <- greater-or-equal click-row, start
+  assert clicked-on-sandboxes?, [extract-sandbox called on click to sandbox editor]
+  {
+    next-sandbox:address:sandbox-data <- get **sandbox, next-sandbox:offset
+    break-unless next-sandbox
+    # if click-row < sandbox.next-sandbox.starting-row-on-screen, break
+    next-start:number <- get *next-sandbox, starting-row-on-screen:offset
+    found?:boolean <- lesser-than click-row, next-start
+    break-if found?
+    sandbox <- get-address **sandbox, next-sandbox:offset
+    loop
+  }
+  # snip sandbox out of its list
+  result:address:sandbox-data <- copy *sandbox
+  *sandbox <- copy next-sandbox
+  # position cursor in sandbox editor
+  sandbox-in-focus?:address:boolean <- get-address *env, sandbox-in-focus?:offset
+  *sandbox-in-focus? <- copy 1/true
+  reply result
+]
diff --git a/edit/007-sandbox-delete.mu b/edit/007-sandbox-delete.mu
new file mode 100644
index 00000000..b4b317e0
--- /dev/null
+++ b/edit/007-sandbox-delete.mu
@@ -0,0 +1,111 @@
+## deleting sandboxes
+
+scenario deleting-sandboxes [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new []
+  2:address:array:character <- new []
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  # run a few commands
+  assume-console [
+    left-click 1, 80
+    type [divide-with-remainder 11, 3]
+    press F4
+    type [add 2, 2]
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊add 2, 2                                         .
+    .                                                  ┊4                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊divide-with-remainder 11, 3                      .
+    .                                                  ┊3                                                .
+    .                                                  ┊2                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+  # delete second sandbox
+  assume-console [
+    left-click 7, 99
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                x.
+    .                                                  ┊add 2, 2                                         .
+    .                                                  ┊4                                                .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+  # delete first sandbox
+  assume-console [
+    left-click 3, 99
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
+
+after <global-touch> [
+  # on a sandbox delete icon? process delete
+  {
+    was-delete?:boolean <- delete-sandbox *t, env
+    break-unless was-delete?
+    hide-screen screen
+    screen <- render-sandbox-side screen, env
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    show-screen screen
+    loop +next-event:label
+  }
+]
+
+# was-deleted?:boolean <- delete-sandbox t:touch-event, env:address:programming-environment-data
+recipe delete-sandbox [
+  local-scope
+  t:touch-event <- next-ingredient
+  env:address:programming-environment-data <- next-ingredient
+  click-column:number <- get t, column:offset
+  current-sandbox:address:editor-data <- get *env, current-sandbox:offset
+  right:number <- get *current-sandbox, right:offset
+  at-right?:boolean <- equal click-column, right
+  reply-unless at-right?, 0/false
+  click-row:number <- get t, row:offset
+  prev:address:address:sandbox-data <- get-address *env, sandbox:offset
+  curr:address:sandbox-data <- get *env, sandbox:offset
+  {
+    break-unless curr
+    # more sandboxes to check
+    {
+      target-row:number <- get *curr, starting-row-on-screen:offset
+      delete-curr?:boolean <- equal target-row, click-row
+      break-unless delete-curr?
+      # delete this sandbox, rerender and stop
+      *prev <- get *curr, next-sandbox:offset
+      reply 1/true
+    }
+    prev <- get-address *curr, next-sandbox:offset
+    curr <- get *curr, next-sandbox:offset
+    loop
+  }
+  reply 0/false
+]
diff --git a/edit/007-sandbox-test.mu b/edit/007-sandbox-test.mu
new file mode 100644
index 00000000..cdad7d3c
--- /dev/null
+++ b/edit/007-sandbox-test.mu
@@ -0,0 +1,173 @@
+## clicking on sandbox results to 'fix' them and turn sandboxes into tests
+
+scenario sandbox-click-on-result-toggles-color-to-green [
+  $close-trace  # trace too long
+  assume-screen 40/width, 10/height
+  # basic recipe
+  1:address:array:character <- new [ 
+recipe foo [
+  add 2, 2
+]]
+  # run it
+  2:address:array:character <- new [foo]
+  assume-console [
+    press F4
+  ]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  event-loop screen:address, console:address, 3:address:programming-environment-data
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  add 2, 2          ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊4                  .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # click on the '4' in the result
+  assume-console [
+    left-click 5, 21
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # color toggles to green
+  screen-should-contain-in-color 2/green, [
+    .                                        .
+    .                                        .
+    .                                        .
+    .                                        .
+    .                                        .
+    .                     4                  .
+    .                                        .
+    .                                        .
+  ]
+  # cursor should remain unmoved
+  run [
+    print-character screen:address, 9251/␣/cursor
+  ]
+  screen-should-contain [
+    .                     run (F4)           .
+    .␣                   ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  add 2, 2          ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊4                  .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # now change the second arg of the 'add'
+  # then rerun
+  assume-console [
+    left-click 3, 11  # cursor to end of line
+    press backspace
+    type [3]
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # result turns red
+  screen-should-contain-in-color 1/red, [
+    .                                        .
+    .                                        .
+    .                                        .
+    .                                        .
+    .                                        .
+    .                     5                  .
+    .                                        .
+    .                                        .
+  ]
+]
+
+# clicks on sandbox responses save it as 'expected'
+after <global-touch> [
+  # right side of screen? check if it's inside the output of any sandbox
+  {
+    sandbox-left-margin:number <- get *current-sandbox, left:offset
+    click-column:number <- get *t, column:offset
+    on-sandbox-side?:boolean <- greater-or-equal click-column, sandbox-left-margin
+    break-unless on-sandbox-side?
+    first-sandbox:address:sandbox-data <- get *env, sandbox:offset
+    break-unless first-sandbox
+    first-sandbox-begins:number <- get *first-sandbox, starting-row-on-screen:offset
+    click-row:number <- get *t, row:offset
+    below-sandbox-editor?:boolean <- greater-or-equal click-row, first-sandbox-begins
+    break-unless below-sandbox-editor?
+    # identify the sandbox whose output is being clicked on
+    sandbox:address:sandbox-data <- find-click-in-sandbox-output env, click-row
+    break-unless sandbox
+    # toggle its expected-response, and save session
+    sandbox <- toggle-expected-response sandbox
+    save-sandboxes env
+    hide-screen screen
+    screen <- render-sandbox-side screen, env, 1/clear
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    # no change in cursor
+    show-screen screen
+    loop +next-event:label
+  }
+]
+
+recipe find-click-in-sandbox-output [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  click-row:number <- next-ingredient
+  # assert click-row >= sandbox.starting-row-on-screen
+  sandbox:address:sandbox-data <- get *env, sandbox:offset
+  start:number <- get *sandbox, starting-row-on-screen:offset
+  clicked-on-sandboxes?:boolean <- greater-or-equal click-row, start
+  assert clicked-on-sandboxes?, [extract-sandbox called on click to sandbox editor]
+  # while click-row < sandbox.next-sandbox.starting-row-on-screen
+  {
+    next-sandbox:address:sandbox-data <- get *sandbox, next-sandbox:offset
+    break-unless next-sandbox
+    next-start:number <- get *next-sandbox, starting-row-on-screen:offset
+    found?:boolean <- lesser-than click-row, next-start
+    break-if found?
+    sandbox <- copy next-sandbox
+    loop
+  }
+  # return sandbox if click is in its output region
+  response-starting-row:number <- get *sandbox, response-starting-row-on-screen:offset
+  reply-unless response-starting-row, 0/no-click-in-sandbox-output
+  click-in-response?:boolean <- greater-or-equal click-row, response-starting-row
+  reply-unless click-in-response?, 0/no-click-in-sandbox-output
+  reply sandbox
+]
+
+recipe toggle-expected-response [
+  local-scope
+  sandbox:address:sandbox-data <- next-ingredient
+  expected-response:address:address:array:character <- get-address *sandbox, expected-response:offset
+  {
+    # if expected-response is set, reset
+    break-unless *expected-response
+    *expected-response <- copy 0
+    reply sandbox/same-as-ingredient:0
+  }
+  # if not, current response is the expected response
+  response:address:array:character <- get *sandbox, response:offset
+  *expected-response <- copy response
+  reply sandbox/same-as-ingredient:0
+]
+
+# when rendering a sandbox, color it in red/green if expected response exists
+after <render-sandbox-response> [
+  {
+    break-unless sandbox-response
+    expected-response:address:array:character <- get *sandbox, expected-response:offset
+    break-unless expected-response  # fall-through to print in grey
+    response-is-expected?:boolean <- string-equal expected-response, sandbox-response
+    {
+      break-if response-is-expected?:boolean
+      row, screen <- render-string screen, sandbox-response, left, right, 1/red, row
+    }
+    {
+      break-unless response-is-expected?:boolean
+      row, screen <- render-string screen, sandbox-response, left, right, 2/green, row
+    }
+    jump +render-sandbox-end:label
+  }
+]
diff --git a/edit/008-sandbox-trace.mu b/edit/008-sandbox-trace.mu
new file mode 100644
index 00000000..7b5dd4e2
--- /dev/null
+++ b/edit/008-sandbox-trace.mu
@@ -0,0 +1,194 @@
+## clicking on the code typed into a sandbox toggles its trace
+
+scenario sandbox-click-on-code-toggles-app-trace [
+  $close-trace  # trace too long
+  assume-screen 40/width, 10/height
+  # basic recipe
+  1:address:array:character <- new [ 
+recipe foo [
+  stash [abc]
+]]
+  # run it
+  2:address:array:character <- new [foo]
+  assume-console [
+    press F4
+  ]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  event-loop screen:address, console:address, 3:address:programming-environment-data
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  stash [abc]       ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # click on the 'foo' line in the sandbox
+  assume-console [
+    left-click 4, 21
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    print-character screen:address, 9251/␣/cursor
+  ]
+  # trace now printed and cursor shouldn't have budged
+  screen-should-contain [
+    .                     run (F4)           .
+    .␣                   ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  stash [abc]       ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊abc                .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  screen-should-contain-in-color 245/grey, [
+    .                                        .
+    .                    ┊                   .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                  x.
+    .                    ┊                   .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊abc                .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # click again on the same region
+  assume-console [
+    left-click 4, 25
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+    print-character screen:address, 9251/␣/cursor
+  ]
+  # trace hidden again
+  screen-should-contain [
+    .                     run (F4)           .
+    .␣                   ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  stash [abc]       ┊                  x.
+    .]                   ┊foo                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+]
+
+scenario sandbox-shows-app-trace-and-result [
+  $close-trace  # trace too long
+  assume-screen 40/width, 10/height
+  # basic recipe
+  1:address:array:character <- new [ 
+recipe foo [
+  stash [abc]
+  add 2, 2
+]]
+  # run it
+  2:address:array:character <- new [foo]
+  assume-console [
+    press F4
+  ]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  event-loop screen:address, console:address, 3:address:programming-environment-data
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  stash [abc]       ┊                  x.
+    .  add 2, 2          ┊foo                .
+    .]                   ┊4                  .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+  # click on the 'foo' line in the sandbox
+  assume-console [
+    left-click 4, 21
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  # trace now printed
+  screen-should-contain [
+    .                     run (F4)           .
+    .                    ┊                   .
+    .recipe foo [        ┊━━━━━━━━━━━━━━━━━━━.
+    .  stash [abc]       ┊                  x.
+    .  add 2, 2          ┊foo                .
+    .]                   ┊abc                .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊4                  .
+    .                    ┊━━━━━━━━━━━━━━━━━━━.
+    .                    ┊                   .
+  ]
+]
+
+# clicks on sandbox code toggle its display-trace? flag
+after <global-touch> [
+  # right side of screen? check if it's inside the code of any sandbox
+  {
+    sandbox-left-margin:number <- get *current-sandbox, left:offset
+    click-column:number <- get *t, column:offset
+    on-sandbox-side?:boolean <- greater-or-equal click-column, sandbox-left-margin
+    break-unless on-sandbox-side?
+    first-sandbox:address:sandbox-data <- get *env, sandbox:offset
+    break-unless first-sandbox
+    first-sandbox-begins:number <- get *first-sandbox, starting-row-on-screen:offset
+    click-row:number <- get *t, row:offset
+    below-sandbox-editor?:boolean <- greater-or-equal click-row, first-sandbox-begins
+    break-unless below-sandbox-editor?
+    # identify the sandbox whose code is being clicked on
+    sandbox:address:sandbox-data <- find-click-in-sandbox-code env, click-row
+    break-unless sandbox
+    # toggle its display-trace? property
+    x:address:boolean <- get-address *sandbox, display-trace?:offset
+    *x <- not *x
+    hide-screen screen
+    screen <- render-sandbox-side screen, env, 1/clear
+    screen <- update-cursor screen, recipes, current-sandbox, *sandbox-in-focus?
+    # no change in cursor
+    show-screen screen
+    loop +next-event:label
+  }
+]
+
+recipe find-click-in-sandbox-code [
+  local-scope
+  env:address:programming-environment-data <- next-ingredient
+  click-row:number <- next-ingredient
+  # assert click-row >= sandbox.starting-row-on-screen
+  sandbox:address:sandbox-data <- get *env, sandbox:offset
+  start:number <- get *sandbox, starting-row-on-screen:offset
+  clicked-on-sandboxes?:boolean <- greater-or-equal click-row, start
+  assert clicked-on-sandboxes?, [extract-sandbox called on click to sandbox editor]
+  # while click-row < sandbox.next-sandbox.starting-row-on-screen
+  {
+    next-sandbox:address:sandbox-data <- get *sandbox, next-sandbox:offset
+    break-unless next-sandbox
+    next-start:number <- get *next-sandbox, starting-row-on-screen:offset
+    found?:boolean <- lesser-than click-row, next-start
+    break-if found?
+    sandbox <- copy next-sandbox
+    loop
+  }
+  # return sandbox if click is in its code region
+  code-ending-row:number <- get *sandbox, code-ending-row-on-screen:offset
+  click-above-response?:boolean <- lesser-or-equal click-row, code-ending-row
+  start:number <- get *sandbox, starting-row-on-screen:offset
+  click-below-menu?:boolean <- greater-than click-row, start
+  click-on-sandbox-code?:boolean <- and click-above-response?, click-below-menu?
+  {
+    break-if click-on-sandbox-code?
+    reply 0/no-click-in-sandbox-output
+  }
+  reply sandbox
+]
+
+# when rendering a sandbox, dump its trace before response/warning if display-trace? property is set
+after <render-sandbox-results> [
+  {
+    display-trace?:boolean <- get *sandbox, display-trace?:offset
+    break-unless display-trace?
+    sandbox-trace:address:array:character <- get *sandbox, trace:offset
+    break-unless sandbox-trace  # nothing to print; move on
+    row, screen <- render-string, screen, sandbox-trace, left, right, 245/grey, row
+    row <- subtract row, 1  # trim the trailing newline that's always present
+  }
+]
diff --git a/edit/009-warnings.mu b/edit/009-warnings.mu
new file mode 100644
index 00000000..b20d930b
--- /dev/null
+++ b/edit/009-warnings.mu
@@ -0,0 +1,199 @@
+## handling malformed programs
+
+scenario run-shows-warnings-in-get [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new [ 
+recipe foo [
+  get 123:number, foo:offset
+]]
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  get 123:number, foo:offset                      ┊                                                 .
+    .]                                                 ┊                                                 .
+    .unknown element foo in container number           ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+  screen-should-contain-in-color 1/red, [
+    .  errors found                                                                                      .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .                                                                                                    .
+    .unknown element foo in container number                                                             .
+    .                                                                                                    .
+  ]
+]
+
+scenario run-shows-missing-type-warnings [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new [ 
+recipe foo [
+  x <- copy 0
+]]
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x <- copy 0                                     ┊                                                 .
+    .]                                                 ┊                                                 .
+    .missing type in 'x <- copy 0'                     ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-shows-unbalanced-bracket-warnings [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  # recipe is incomplete (unbalanced '[')
+  1:address:array:character <- new [ 
+recipe foo «
+  x <- copy 0
+]
+  string-replace 1:address:array:character, 171/«, 91  # '['
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo \\\[                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x <- copy 0                                     ┊                                                 .
+    .                                                  ┊                                                 .
+    .9: unbalanced '\\\[' for recipe                      ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-shows-get-on-non-container-warnings [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new [ 
+recipe foo [
+  x:address:point <- new point:type
+  get x:address:point, 1:offset
+]]
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .                                                                                 run (F4)           .
+    .                                                  ┊                                                 .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x:address:point <- new point:type               ┊                                                x.
+    .  get x:address:point, 1:offset                   ┊foo                                              .
+    .]                                                 ┊foo: first ingredient of 'get' should be a conta↩.
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊iner, but got x:address:point                    .
+    .                                                  ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-shows-non-literal-get-argument-warnings [
+  $close-trace  # trace too long
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new [ 
+recipe foo [
+  x:number <- copy 0
+  y:address:point <- new point:type
+  get *y:address:point, x:number
+]]
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x:number <- copy 0                              ┊                                                 .
+    .  y:address:point <- new point:type               ┊                                                 .
+    .  get *y:address:point, x:number                  ┊                                                 .
+    .]                                                 ┊                                                 .
+    .foo: expected ingredient 1 of 'get' to have type ↩┊                                                 .
+    .'offset'; got x:number                            ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
+
+scenario run-shows-warnings-everytime [
+  $close-trace  # trace too long
+  # try to run a file with an error
+  assume-screen 100/width, 15/height
+  1:address:array:character <- new [ 
+recipe foo [
+  x:number <- copy y:number
+]]
+  2:address:array:character <- new [foo]
+  3:address:programming-environment-data <- new-programming-environment screen:address, 1:address:array:character, 2:address:array:character
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x:number <- copy y:number                       ┊                                                 .
+    .]                                                 ┊                                                 .
+    .use before set: y in foo                          ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+  # rerun the file, check for the same error
+  assume-console [
+    press F4
+  ]
+  run [
+    event-loop screen:address, console:address, 3:address:programming-environment-data
+  ]
+  screen-should-contain [
+    .  errors found                                                                   run (F4)           .
+    .                                                  ┊foo                                              .
+    .recipe foo [                                      ┊━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━.
+    .  x:number <- copy y:number                       ┊                                                 .
+    .]                                                 ┊                                                 .
+    .use before set: y in foo                          ┊                                                 .
+    .┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┊                                                 .
+    .                                                  ┊                                                 .
+  ]
+]
diff --git a/edit/010-undo.mu b/edit/010-undo.mu
new file mode 100644
index 00000000..41450f26
--- /dev/null
+++ b/edit/010-undo.mu
@@ -0,0 +1,2056 @@
+## undo/redo
+
+# for every undoable event, create a type of *operation* that contains all the
+# information needed to reverse it
+exclusive-container operation [
+  typing:insert-operation
+  move:move-operation
+  delete:delete-operation
+]
+
+container insert-operation [
+  before-row:number
+  before-column:number
+  before-top-of-screen:address:duplex-list:character
+  after-row:number
+  after-column:number
+  after-top-of-screen:address:duplex-list:character
+  # inserted text is from 'insert-from' until 'insert-until'; list doesn't have to terminate
+  insert-from:address:duplex-list:character
+  insert-until:address:duplex-list:character
+  tag:number  # event causing this operation; might be used to coalesce runs of similar events
+    # 0: no coalesce (enter+indent)
+    # 1: regular alphanumeric characters
+]
+
+container move-operation [
+  before-row:number
+  before-column:number
+  before-top-of-screen:address:duplex-list:character
+  after-row:number
+  after-column:number
+  after-top-of-screen:address:duplex-list:character
+  tag:number  # event causing this operation; might be used to coalesce runs of similar events
+    # 0: no coalesce (touch events, etc)
+    # 1: left arrow
+    # 2: right arrow
+    # 3: up arrow
+    # 4: down arrow
+]
+
+container delete-operation [
+  before-row:number
+  before-column:number
+  before-top-of-screen:address:duplex-list:character
+  after-row:number
+  after-column:number
+  after-top-of-screen:address:duplex-list:character
+  deleted-text:address:duplex-list:character
+  delete-from:address:duplex-list:character
+  delete-until:address:duplex-list:character
+  tag:number  # event causing this operation; might be used to coalesce runs of similar events
+    # 0: no coalesce (ctrl-k, ctrl-u)
+    # 1: backspace
+    # 2: delete
+]
+
+# every editor accumulates a list of operations to undo/redo
+container editor-data [
+  undo:address:list:address:operation
+  redo:address:list:address:operation
+]
+
+# ctrl-z - undo operation
+after <handle-special-character> [
+  {
+    undo?:boolean <- equal *c, 26/ctrl-z
+    break-unless undo?
+    undo:address:address:list <- get-address *editor, undo:offset
+    break-unless *undo
+    op:address:operation <- first *undo
+    *undo <- rest *undo
+    redo:address:address:list <- get-address *editor, redo:offset
+    *redo <- push op, *redo
+    <handle-undo>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+# ctrl-y - redo operation
+after <handle-special-character> [
+  {
+    redo?:boolean <- equal *c, 25/ctrl-y
+    break-unless redo?
+    redo:address:address:list <- get-address *editor, redo:offset
+    break-unless *redo
+    op:address:operation <- first *redo
+    *redo <- rest *redo
+    undo:address:address:list <- get-address *editor, undo:offset
+    *undo <- push op, *undo
+    <handle-redo>
+    reply screen/same-as-ingredient:0, editor/same-as-ingredient:1, 1/go-render
+  }
+]
+
+# undo typing
+
+scenario editor-can-undo-typing [
+  # create an editor and type a character
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [0]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # character should be gone
+  screen-should-contain [
+    .          .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .1         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# save operation to undo
+after <insert-character-begin> [
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+  cursor-before:address:duplex-list <- copy *before-cursor
+]
+before <insert-character-end> [
+  top-after:address:duplex-list <- get *editor, top-of-screen:offset
+  undo:address:address:list <- get-address *editor, undo:offset
+  {
+    # if previous operation was an insert, coalesce this operation with it
+    break-unless *undo
+    op:address:operation <- first *undo
+    typing:address:insert-operation <- maybe-convert *op, typing:variant
+    break-unless typing
+    previous-coalesce-tag:number <- get *typing, tag:offset
+    break-unless previous-coalesce-tag
+    insert-until:address:address:duplex-list <- get-address *typing, insert-until:offset
+    *insert-until <- next-duplex *before-cursor
+    after-row:address:number <- get-address *typing, after-row:offset
+    *after-row <- copy *cursor-row
+    after-column:address:number <- get-address *typing, after-column:offset
+    *after-column <- copy *cursor-column
+    after-top:address:number <- get-address *typing, after-top-of-screen:offset
+    *after-top <- get *editor, top-of-screen:offset
+    break +done-adding-insert-operation:label
+  }
+  # if not, create a new operation
+  insert-from:address:duplex-list <- next-duplex cursor-before
+  insert-to:address:duplex-list <- next-duplex insert-from
+  op:address:operation <- new operation:type
+  *op <- merge 0/insert-operation, save-row/before, save-column/before, top-before, *cursor-row/after, *cursor-column/after, top-after, insert-from, insert-to, 1/coalesce
+  editor <- add-operation editor, op
+  +done-adding-insert-operation
+]
+
+# enter operations never coalesce with typing before or after
+after <insert-enter-begin> [
+  cursor-row-before:number <- copy *cursor-row
+  cursor-column-before:number <- copy *cursor-column
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+  cursor-before:address:duplex-list <- copy *before-cursor
+]
+before <insert-enter-end> [
+  top-after:address:duplex-list <- get *editor, top-of-screen:offset
+  # never coalesce
+  insert-from:address:duplex-list <- next-duplex cursor-before
+  insert-to:address:duplex-list <- next-duplex *before-cursor
+  op:address:operation <- new operation:type
+  *op <- merge 0/insert-operation, cursor-row-before, cursor-column-before, top-before, *cursor-row/after, *cursor-column/after, top-after, insert-from, insert-to, 0/never-coalesce
+  editor <- add-operation editor, op
+]
+
+# Everytime you add a new operation to the undo stack, be sure to clear the
+# redo stack, because it's now obsolete.
+# Beware: since we're counting cursor moves as operations, this means just
+# moving the cursor can lose work on the undo stack.
+recipe add-operation [
+  local-scope
+  editor:address:editor-data <- next-ingredient
+  op:address:operation <- next-ingredient
+  undo:address:address:list:address:operation <- get-address *editor, undo:offset
+  *undo <- push op *undo
+  redo:address:address:list:address:operation <- get-address *editor, redo:offset
+  *redo <- copy 0
+  reply editor/same-as-ingredient:0
+]
+
+after <handle-undo> [
+  {
+    typing:address:insert-operation <- maybe-convert *op, typing:variant
+    break-unless typing
+    start:address:duplex-list <- get *typing, insert-from:offset
+    end:address:duplex-list <- get *typing, insert-until:offset
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    *before-cursor <- prev-duplex start
+    remove-duplex-between *before-cursor, end
+    *cursor-row <- get *typing, before-row:offset
+    *cursor-column <- get *typing, before-column:offset
+    top:address:address:duplex-list <- get *editor, top-of-screen:offset
+    *top <- get *typing, before-top-of-screen:offset
+  }
+]
+
+scenario editor-can-undo-typing-multiple [
+  # create an editor and type multiple characters
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [012]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # all characters must be gone
+  screen-should-contain [
+    .          .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-can-undo-typing-multiple-2 [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # type some characters
+  assume-console [
+    type [012]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .012a      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # back to original text
+  screen-should-contain [
+    .          .
+    .a         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [3]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .3a        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-can-undo-typing-enter [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [  abc]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # new line
+  assume-console [
+    left-click 1, 8
+    press enter
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .  abc     .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # line is indented
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 2
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 5
+  ]
+  # back to original text
+  screen-should-contain [
+    .          .
+    .  abc     .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # cursor should be at end of line
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .  abc1    .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# redo typing
+
+scenario editor-redo-typing [
+  # create an editor, type something, undo
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [012]
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .a         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # all characters must be back
+  screen-should-contain [
+    .          .
+    .012a      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [3]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .0123a     .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <handle-redo> [
+  {
+    typing:address:insert-operation <- maybe-convert *op, typing:variant
+    break-unless typing
+    insert-from:address:duplex-list <- get *typing, insert-from:offset  # ignore insert-to because it's already been spliced away
+    # assert insert-to matches next-duplex(*before-cursor)
+    insert-duplex-range *before-cursor, insert-from
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    *cursor-row <- get *typing, after-row:offset
+    *cursor-column <- get *typing, after-column:offset
+    top:address:address:duplex-list <- get *editor, top-of-screen:offset
+    *top <- get *typing, after-top-of-screen:offset
+  }
+]
+
+scenario editor-redo-typing-empty [
+  # create an editor, type something, undo
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [012]
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # all characters must be back
+  screen-should-contain [
+    .          .
+    .012       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [3]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .0123      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+scenario editor-work-clears-redo-stack [
+  # create an editor with some text, do some work, undo
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [1]
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # do some more work
+  assume-console [
+    type [0]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .0abc      .
+    .def       .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # nothing should happen
+  screen-should-contain [
+    .          .
+    .0abc      .
+    .def       .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-redo-typing-and-enter-and-tab [
+  # create an editor
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # insert some text and tabs, hit enter, some more text and tabs
+  assume-console [
+    press tab
+    type [ab]
+    press tab
+    type [cd]
+    press enter
+    press tab
+    type [efg]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .    efg   .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 7
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # typing in second line deleted, but not indent
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 2
+  ]
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # undo again
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # indent and newline deleted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 8
+  ]
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # undo again
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # empty screen
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  screen-should-contain [
+    .          .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 8
+  ]
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo again
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # newline and indent inserted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 2
+  ]
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo again
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # indent and newline deleted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 7
+  ]
+  screen-should-contain [
+    .          .
+    .  ab  cd  .
+    .    efg   .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# undo cursor movement and scroll
+
+scenario editor-can-undo-touch [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor
+  assume-console [
+    left-click 3, 1
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # click undone
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .1abc      .
+    .def       .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+after <move-cursor-begin> [
+  before-cursor-row:number <- get *editor, cursor-row:offset
+  before-cursor-column:number <- get *editor, cursor-column:offset
+  before-top-of-screen:address:duplex-list <- get *editor, top-of-screen:offset
+]
+before <move-cursor-end> [
+  after-cursor-row:number <- get *editor, cursor-row:offset
+  after-cursor-column:number <- get *editor, cursor-column:offset
+  after-top-of-screen:address:duplex-list <- get *editor, top-of-screen:offset
+  {
+    break-unless undo-coalesce-tag
+    # if previous operation was also a move, and also had the same coalesce
+    # tag, coalesce with it
+    undo:address:address:list <- get-address *editor, undo:offset
+    break-unless *undo
+    op:address:operation <- first *undo
+    move:address:move-operation <- maybe-convert *op, move:variant
+    break-unless move
+    previous-coalesce-tag:number <- get *move, tag:offset
+    coalesce?:boolean <- equal undo-coalesce-tag, previous-coalesce-tag
+    break-unless coalesce?
+    after-row:address:number <- get-address *move, after-row:offset
+    *after-row <- copy after-cursor-row
+    after-column:address:number <- get-address *move, after-column:offset
+    *after-column <- copy after-cursor-column
+    after-top:address:number <- get-address *move, after-top-of-screen:offset
+    *after-top <- get *editor, top-of-screen:offset
+    break +done-adding-move-operation:label
+  }
+  op:address:operation <- new operation:type
+  *op <- merge 1/move-operation, before-cursor-row, before-cursor-column, before-top-of-screen, after-cursor-row, after-cursor-column, after-top-of-screen, undo-coalesce-tag
+  editor <- add-operation editor, op
+  +done-adding-move-operation
+]
+
+after <handle-undo> [
+  {
+    move:address:move-operation <- maybe-convert *op, move:variant
+    break-unless move
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    top:address:address:duplex-list <- get-address *editor, top-of-screen:offset
+    *cursor-row <- get *move, before-row:offset
+    *cursor-column <- get *move, before-column:offset
+    *top <- get *move, before-top-of-screen:offset
+  }
+]
+
+scenario editor-can-undo-scroll [
+  # screen has 1 line for menu + 3 lines
+  assume-screen 5/width, 4/height
+  # editor contains a wrapped line
+  1:address:array:character <- new [a
+b
+cdefgh]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 5/right
+  # position cursor at end of screen and try to move right
+  assume-console [
+    left-click 3, 3
+    press right-arrow
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  # screen scrolls
+  screen-should-contain [
+    .     .
+    .b    .
+    .cdef↩.
+    .gh   .
+  ]
+  memory-should-contain [
+    3 <- 3
+    4 <- 0
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moved back
+  memory-should-contain [
+    3 <- 3
+    4 <- 3
+  ]
+  # scroll undone
+  screen-should-contain [
+    .     .
+    .a    .
+    .b    .
+    .cdef↩.
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .     .
+    .b    .
+    .cde1↩.
+    .fgh  .
+  ]
+]
+
+scenario editor-can-undo-left-arrow [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor
+  assume-console [
+    left-click 3, 1
+    press left-arrow
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 3
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .g1hi      .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-up-arrow [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor
+  assume-console [
+    left-click 3, 1
+    press up-arrow
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 3
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .g1hi      .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-down-arrow [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor
+  assume-console [
+    left-click 2, 1
+    press down-arrow
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .d1ef      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-ctrl-f [
+  # create an editor with multiple pages of text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # scroll the page
+  assume-console [
+    press ctrl-f
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen should again show page 1
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+    .d         .
+  ]
+]
+
+scenario editor-can-undo-page-down [
+  # create an editor with multiple pages of text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # scroll the page
+  assume-console [
+    press page-down
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen should again show page 1
+  screen-should-contain [
+    .          .
+    .a         .
+    .b         .
+    .c         .
+    .d         .
+  ]
+]
+
+scenario editor-can-undo-ctrl-b [
+  # create an editor with multiple pages of text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # scroll the page down and up
+  assume-console [
+    press page-down
+    press ctrl-b
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen should again show page 2
+  screen-should-contain [
+    .          .
+    .d         .
+    .e         .
+    .f         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-page-up [
+  # create an editor with multiple pages of text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [a
+b
+c
+d
+e
+f]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # scroll the page down and up
+  assume-console [
+    press page-down
+    press page-up
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen should again show page 2
+  screen-should-contain [
+    .          .
+    .d         .
+    .e         .
+    .f         .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-ctrl-a [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor, then to start of line
+  assume-console [
+    left-click 2, 1
+    press ctrl-a
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .d1ef      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-home [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor, then to start of line
+  assume-console [
+    left-click 2, 1
+    press home
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .d1ef      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-ctrl-e [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor, then to start of line
+  assume-console [
+    left-click 2, 1
+    press ctrl-e
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .d1ef      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-can-undo-end [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor, then to start of line
+  assume-console [
+    left-click 2, 1
+    press end
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves back
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .d1ef      .
+    .ghi       .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+scenario editor-separates-undo-insert-from-undo-cursor-move [
+  # create an editor, type some text, move the cursor, type some more text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    type [abc]
+    left-click 1, 1
+    type [d]
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  screen-should-contain [
+    .          .
+    .adbc      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # last letter typed is deleted
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # undo again
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # no change to screen; cursor moves
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  # undo again
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # screen empty
+  screen-should-contain [
+    .          .
+    .          .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # first insert
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  # redo again
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # redo again
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # second insert
+  screen-should-contain [
+    .          .
+    .adbc      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+]
+
+scenario editor-can-undo-multiple-arrows-in-the-same-direction [
+  # create an editor with some text
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # move the cursor
+  assume-console [
+    left-click 2, 1
+    press right-arrow
+    press right-arrow
+    press up-arrow
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # up-arrow is undone
+  memory-should-contain [
+    3 <- 2
+    4 <- 3
+  ]
+  # undo again
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # both right-arrows are undone
+  memory-should-contain [
+    3 <- 2
+    4 <- 1
+  ]
+]
+
+# redo cursor movement and scroll
+
+scenario editor-redo-touch [
+  # create an editor with some text, click on a character, undo
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def
+ghi]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  assume-console [
+    left-click 3, 1
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+    3:number <- get *2:address:editor-data, cursor-row:offset
+    4:number <- get *2:address:editor-data, cursor-column:offset
+  ]
+  # cursor moves to left-click
+  memory-should-contain [
+    3 <- 3
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .g1hi      .
+    .┈┈┈┈┈┈┈┈┈┈.
+  ]
+]
+
+after <handle-redo> [
+  {
+    move:address:move-operation <- maybe-convert *op, move:variant
+    break-unless move
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    *cursor-row <- get *move, after-row:offset
+    *cursor-column <- get *move, after-column:offset
+    top:address:address:duplex-list <- get *editor, top-of-screen:offset
+    *top <- get *move, after-top-of-screen:offset
+  }
+]
+
+# undo backspace
+
+scenario editor-can-undo-and-redo-backspace [
+  # create an editor
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # insert some text and hit backspace
+  assume-console [
+    type [abc]
+    press backspace
+    press backspace
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .a         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 3
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  screen-should-contain [
+    .          .
+    .a         .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+# save operation to undo
+after <backspace-character-begin> [
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+]
+before <backspace-character-end> [
+  {
+    break-unless backspaced-cell  # backspace failed; don't add an undo operation
+    top-after:address:duplex-list <- get *editor, top-of-screen:offset
+    undo:address:address:list <- get-address *editor, undo:offset
+    {
+      # if previous operation was an insert, coalesce this operation with it
+      break-unless *undo
+      op:address:operation <- first *undo
+      deletion:address:delete-operation <- maybe-convert *op, delete:variant
+      break-unless deletion
+      previous-coalesce-tag:number <- get *deletion, tag:offset
+      coalesce?:boolean <- equal previous-coalesce-tag, 1/coalesce-backspace
+      break-unless coalesce?
+      delete-from:address:address:duplex-list <- get-address *deletion, delete-from:offset
+      *delete-from <- copy *before-cursor
+      backspaced-so-far:address:address:duplex-list <- get-address *deletion, deleted-text:offset
+      insert-duplex-range backspaced-cell, *backspaced-so-far
+      *backspaced-so-far <- copy backspaced-cell
+      after-row:address:number <- get-address *deletion, after-row:offset
+      *after-row <- copy *cursor-row
+      after-column:address:number <- get-address *deletion, after-column:offset
+      *after-column <- copy *cursor-column
+      after-top:address:number <- get-address *deletion, after-top-of-screen:offset
+      *after-top <- get *editor, top-of-screen:offset
+      break +done-adding-backspace-operation:label
+    }
+    # if not, create a new operation
+    op:address:operation <- new operation:type
+    deleted-until:address:duplex-list <- next-duplex *before-cursor
+    *op <- merge 2/delete-operation, save-row/before, save-column/before, top-before, *cursor-row/after, *cursor-column/after, top-after, backspaced-cell/deleted, *before-cursor/delete-from, deleted-until, 1/coalesce-backspace
+    editor <- add-operation editor, op
+    +done-adding-backspace-operation
+  }
+]
+
+after <handle-undo> [
+  {
+    deletion:address:delete-operation <- maybe-convert *op, delete:variant
+    break-unless deletion
+    start2:address:address:duplex-list <- get-address *editor, data:offset
+    anchor:address:duplex-list <- get *deletion, delete-from:offset
+    break-unless anchor
+    deleted:address:duplex-list <- get *deletion, deleted-text:offset
+    old-cursor:address:duplex-list <- last-duplex deleted
+    insert-duplex-range anchor, deleted
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    *before-cursor <- copy old-cursor
+    *cursor-row <- get *deletion, before-row:offset
+    *cursor-column <- get *deletion, before-column:offset
+    top:address:address:duplex-list <- get *editor, top-of-screen:offset
+    *top <- get *deletion, before-top-of-screen:offset
+  }
+]
+
+after <handle-redo> [
+  {
+    deletion:address:delete-operation <- maybe-convert *op, delete:variant
+    break-unless deletion
+    start:address:duplex-list <- get *deletion, delete-from:offset
+    end:address:duplex-list <- get *deletion, delete-until:offset
+    remove-duplex-between start, end
+    # assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen
+    *cursor-row <- get *deletion, after-row:offset
+    *cursor-column <- get *deletion, after-column:offset
+    top:address:address:duplex-list <- get *editor, top-of-screen:offset
+    *top <- get *deletion, after-top-of-screen:offset
+  }
+]
+
+# undo delete
+
+scenario editor-can-undo-and-redo-delete [
+  # create an editor
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new []
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # insert some text and hit delete and backspace a few times
+  assume-console [
+    type [abcdef]
+    left-click 1, 2
+    press delete
+    press backspace
+    press delete
+    press delete
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .af        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # undo deletes
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  screen-should-contain [
+    .          .
+    .adef      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # undo backspace
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  screen-should-contain [
+    .          .
+    .abdef     .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # undo first delete
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  screen-should-contain [
+    .          .
+    .abcdef    .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo first delete
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  screen-should-contain [
+    .          .
+    .abdef     .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo backspace
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  screen-should-contain [
+    .          .
+    .adef      .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  # redo deletes
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  screen-should-contain [
+    .          .
+    .af        .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <delete-character-begin> [
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+]
+before <delete-character-end> [
+  {
+    break-unless deleted-cell  # delete failed; don't add an undo operation
+    top-after:address:duplex-list <- get *editor, top-of-screen:offset
+    undo:address:address:list <- get-address *editor, undo:offset
+    {
+      # if previous operation was an insert, coalesce this operation with it
+      break-unless *undo
+      op:address:operation <- first *undo
+      deletion:address:delete-operation <- maybe-convert *op, delete:variant
+      break-unless deletion
+      previous-coalesce-tag:number <- get *deletion, tag:offset
+      coalesce?:boolean <- equal previous-coalesce-tag, 2/coalesce-delete
+      break-unless coalesce?
+      delete-until:address:address:duplex-list <- get-address *deletion, delete-until:offset
+      *delete-until <- next-duplex *before-cursor
+      deleted-so-far:address:address:duplex-list <- get-address *deletion, deleted-text:offset
+      *deleted-so-far <- append-duplex *deleted-so-far, deleted-cell
+      after-row:address:number <- get-address *deletion, after-row:offset
+      *after-row <- copy *cursor-row
+      after-column:address:number <- get-address *deletion, after-column:offset
+      *after-column <- copy *cursor-column
+      after-top:address:number <- get-address *deletion, after-top-of-screen:offset
+      *after-top <- get *editor, top-of-screen:offset
+      break +done-adding-delete-operation:label
+    }
+    # if not, create a new operation
+    op:address:operation <- new operation:type
+    deleted-until:address:duplex-list <- next-duplex *before-cursor
+    *op <- merge 2/delete-operation, save-row/before, save-column/before, top-before, *cursor-row/after, *cursor-column/after, top-after, deleted-cell/deleted, *before-cursor/delete-from, deleted-until, 2/coalesce-delete
+    editor <- add-operation editor, op
+    +done-adding-delete-operation
+  }
+]
+
+# undo ctrl-k
+
+scenario editor-can-undo-and-redo-ctrl-k [
+  # create an editor
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # insert some text and hit delete and backspace a few times
+  assume-console [
+    left-click 1, 1
+    press ctrl-k
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .a         .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  screen-should-contain [
+    .          .
+    .a         .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 1
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .a1        .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <delete-to-end-of-line-begin> [
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+]
+before <delete-to-end-of-line-end> [
+  {
+    break-unless deleted-cells  # delete failed; don't add an undo operation
+    top-after:address:duplex-list <- get *editor, top-of-screen:offset
+    undo:address:address:list <- get-address *editor, undo:offset
+    op:address:operation <- new operation:type
+    deleted-until:address:duplex-list <- next-duplex *before-cursor
+    *op <- merge 2/delete-operation, save-row/before, save-column/before, top-before, *cursor-row/after, *cursor-column/after, top-after, deleted-cells/deleted, *before-cursor/delete-from, deleted-until, 0/never-coalesce
+    editor <- add-operation editor, op
+    +done-adding-delete-operation
+  }
+]
+
+# undo ctrl-u
+
+scenario editor-can-undo-and-redo-ctrl-u [
+  # create an editor
+  assume-screen 10/width, 5/height
+  1:address:array:character <- new [abc
+def]
+  2:address:editor-data <- new-editor 1:address:array:character, screen:address, 0/left, 10/right
+  editor-render screen, 2:address:editor-data
+  # insert some text and hit delete and backspace a few times
+  assume-console [
+    left-click 1, 2
+    press ctrl-u
+  ]
+  editor-event-loop screen:address, console:address, 2:address:editor-data
+  screen-should-contain [
+    .          .
+    .c         .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  # undo
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .abc       .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 2
+  ]
+  # redo
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  # first line inserted
+  screen-should-contain [
+    .          .
+    .c         .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+  3:number <- get *2:address:editor-data, cursor-row:offset
+  4:number <- get *2:address:editor-data, cursor-column:offset
+  memory-should-contain [
+    3 <- 1
+    4 <- 0
+  ]
+  # cursor should be in the right place
+  assume-console [
+    type [1]
+  ]
+  run [
+    editor-event-loop screen:address, console:address, 2:address:editor-data
+  ]
+  screen-should-contain [
+    .          .
+    .1c        .
+    .def       .
+    .┈┈┈┈┈┈┈┈┈┈.
+    .          .
+  ]
+]
+
+after <delete-to-start-of-line-begin> [
+  top-before:address:duplex-list <- get *editor, top-of-screen:offset
+]
+before <delete-to-start-of-line-end> [
+  {
+    break-unless deleted-cells  # delete failed; don't add an undo operation
+    top-after:address:duplex-list <- get *editor, top-of-screen:offset
+    undo:address:address:list <- get-address *editor, undo:offset
+    op:address:operation <- new operation:type
+    deleted-until:address:duplex-list <- next-duplex *before-cursor
+    *op <- merge 2/delete-operation, save-row/before, save-column/before, top-before, *cursor-row/after, *cursor-column/after, top-after, deleted-cells/deleted, *before-cursor/delete-from, deleted-until, 0/never-coalesce
+    editor <- add-operation editor, op
+    +done-adding-delete-operation
+  }
+]
+
+# todo:
+# operations for recipe side and each sandbox-data
+# undo delete sandbox as a separate primitive on the status bar
diff --git a/edit/Readme b/edit/Readme
new file mode 100644
index 00000000..d32fdf66
--- /dev/null
+++ b/edit/Readme
@@ -0,0 +1 @@
+Environment for learning programming using mu: http://akkartik.name/post/mu