2177

author: Kartik K. Agaram <vc@akkartik.com> 2015-09-07 10:37:27 -0700
committer: Kartik K. Agaram <vc@akkartik.com> 2015-09-07 10:37:27 -0700
commit: f5465e1220d73e237c51897b7d1211ec53b0dc04 (patch)
tree: 939ee8e57241b8515aede8106c6420e330ace75a /html/edit/011-editor-undo.mu.html
parent: 5ccf2653fb7d31b013f77df4e92e964e45c54f8a (diff)
download: mu-f5465e1220d73e237c51897b7d1211ec53b0dc04.tar.gz
1 files changed, 2095 insertions, 0 deletions
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
+<html>
+<head>
+<meta http-equiv="content-type" content="text/html; charset=UTF-8">
+<title>Mu - edit/011-editor-undo.mu</title>
+<meta name="Generator" content="Vim/7.4">
+<meta name="plugin-version" content="vim7.4_v1">
+<meta name="syntax" content="none">
+<meta name="settings" content="use_css,pre_wrap,no_foldcolumn,expand_tabs,prevent_copy=">
+<meta name="colorscheme" content="minimal">
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+<!--
+pre { white-space: pre-wrap; font-family: monospace; color: #eeeeee; background-color: #080808; }
+body { font-family: monospace; color: #eeeeee; background-color: #080808; }
+* { font-size: 1.05em; }
+.muRecipe { color: #ff8700; }
+.muData { color: #ffff00; }
+.Special { color: #ff6060; }
+.muScenario { color: #00af00; }
+.Comment { color: #9090ff; }
+.Constant { color: #00a0a0; }
+.SalientComment { color: #00ffff; }
+.Delimiter { color: #a04060; }
+.muControl { color: #c0a020; }
+-->
+</style>
+
+<script type='text/javascript'>
+<!--
+
+-->
+</script>
+</head>
+<body>
+<pre id='vimCodeElement'>
+<span class="SalientComment">## undo/redo</span>
+
+<span class="Comment"># for every undoable event, create a type of *operation* that contains all the</span>
+<span class="Comment"># information needed to reverse it</span>
+<span class="muData">exclusive-container</span> operation [
+  typing:insert-operation
+  move:move-operation
+  delete:delete-operation
+]
+
+<span class="muData">container</span> 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
+  <span class="Comment"># inserted text is from 'insert-from' until 'insert-until'; list doesn't have to terminate</span>
+  insert-from:address:duplex-list:character
+  insert-until:address:duplex-list:character
+  tag:number  <span class="Comment"># event causing this operation; might be used to coalesce runs of similar events</span>
+    <span class="Comment"># 0: no coalesce (enter+indent)</span>
+    <span class="Comment"># 1: regular alphanumeric characters</span>
+]
+
+<span class="muData">container</span> 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  <span class="Comment"># event causing this operation; might be used to coalesce runs of similar events</span>
+    <span class="Comment"># 0: no coalesce (touch events, etc)</span>
+    <span class="Comment"># 1: left arrow</span>
+    <span class="Comment"># 2: right arrow</span>
+    <span class="Comment"># 3: up arrow</span>
+    <span class="Comment"># 4: down arrow</span>
+]
+
+<span class="muData">container</span> 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  <span class="Comment"># event causing this operation; might be used to coalesce runs of similar events</span>
+    <span class="Comment"># 0: no coalesce (ctrl-k, ctrl-u)</span>
+    <span class="Comment"># 1: backspace</span>
+    <span class="Comment"># 2: delete</span>
+]
+
+<span class="Comment"># every editor accumulates a list of operations to undo/redo</span>
+<span class="muData">container</span> editor-data [
+  undo:address:list:address:operation
+  redo:address:list:address:operation
+]
+
+<span class="Comment"># ctrl-z - undo operation</span>
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-special-character&gt;</span> [
+  <span class="Delimiter">{</span>
+    undo?:boolean<span class="Special"> &lt;- </span>equal *c, <span class="Constant">26/ctrl-z</span>
+    <span class="muControl">break-unless</span> undo?
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    <span class="muControl">break-unless</span> *undo
+    op:address:operation<span class="Special"> &lt;- </span>first *undo
+    *undo<span class="Special"> &lt;- </span>rest *undo
+    redo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">redo:offset</span>
+    *redo<span class="Special"> &lt;- </span>push op, *redo
+<span class="Constant">    &lt;handle-undo&gt;</span>
+    <span class="muControl">reply</span> screen/same-as-ingredient:<span class="Constant">0</span>, editor/same-as-ingredient:<span class="Constant">1</span>, <span class="Constant">1/go-render</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># ctrl-y - redo operation</span>
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-special-character&gt;</span> [
+  <span class="Delimiter">{</span>
+    redo?:boolean<span class="Special"> &lt;- </span>equal *c, <span class="Constant">25/ctrl-y</span>
+    <span class="muControl">break-unless</span> redo?
+    redo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">redo:offset</span>
+    <span class="muControl">break-unless</span> *redo
+    op:address:operation<span class="Special"> &lt;- </span>first *redo
+    *redo<span class="Special"> &lt;- </span>rest *redo
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    *undo<span class="Special"> &lt;- </span>push op, *undo
+<span class="Constant">    &lt;handle-redo&gt;</span>
+    <span class="muControl">reply</span> screen/same-as-ingredient:<span class="Constant">0</span>, editor/same-as-ingredient:<span class="Constant">1</span>, <span class="Constant">1/go-render</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># undo typing</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-typing [
+  <span class="Comment"># create an editor and type a character</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[0]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># character should be gone</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .1         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="Comment"># save operation to undo</span>
+<span class="muRecipe">after</span> <span class="Constant">&lt;insert-character-begin&gt;</span> [
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+  cursor-before:address:duplex-list<span class="Special"> &lt;- </span>copy *before-cursor
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;insert-character-end&gt;</span> [
+  top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+  undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+  <span class="Delimiter">{</span>
+    <span class="Comment"># if previous operation was an insert, coalesce this operation with it</span>
+    <span class="muControl">break-unless</span> *undo
+    op:address:operation<span class="Special"> &lt;- </span>first *undo
+    typing:address:insert-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">typing:variant</span>
+    <span class="muControl">break-unless</span> typing
+    previous-coalesce-tag:number<span class="Special"> &lt;- </span>get *typing, <span class="Constant">tag:offset</span>
+    <span class="muControl">break-unless</span> previous-coalesce-tag
+    insert-until:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *typing, <span class="Constant">insert-until:offset</span>
+    *insert-until<span class="Special"> &lt;- </span>next-duplex *before-cursor
+    after-row:address:number<span class="Special"> &lt;- </span>get-address *typing, <span class="Constant">after-row:offset</span>
+    *after-row<span class="Special"> &lt;- </span>copy *cursor-row
+    after-column:address:number<span class="Special"> &lt;- </span>get-address *typing, <span class="Constant">after-column:offset</span>
+    *after-column<span class="Special"> &lt;- </span>copy *cursor-column
+    after-top:address:number<span class="Special"> &lt;- </span>get-address *typing, <span class="Constant">after-top-of-screen:offset</span>
+    *after-top<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    <span class="muControl">break</span> <span class="Constant">+done-adding-insert-operation:label</span>
+  <span class="Delimiter">}</span>
+  <span class="Comment"># if not, create a new operation</span>
+  insert-from:address:duplex-list<span class="Special"> &lt;- </span>next-duplex cursor-before
+  insert-to:address:duplex-list<span class="Special"> &lt;- </span>next-duplex insert-from
+  op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+  *op<span class="Special"> &lt;- </span>merge <span class="Constant">0/insert-operation</span>, save-row/<span class="muRecipe">before</span>, save-column/<span class="muRecipe">before</span>, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, insert-from, insert-to, <span class="Constant">1/coalesce</span>
+  editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">  +done-adding-insert-operation</span>
+]
+
+<span class="Comment"># enter operations never coalesce with typing before or after</span>
+<span class="muRecipe">after</span> <span class="Constant">&lt;insert-enter-begin&gt;</span> [
+  cursor-row-before:number<span class="Special"> &lt;- </span>copy *cursor-row
+  cursor-column-before:number<span class="Special"> &lt;- </span>copy *cursor-column
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+  cursor-before:address:duplex-list<span class="Special"> &lt;- </span>copy *before-cursor
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;insert-enter-end&gt;</span> [
+  top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+  <span class="Comment"># never coalesce</span>
+  insert-from:address:duplex-list<span class="Special"> &lt;- </span>next-duplex cursor-before
+  insert-to:address:duplex-list<span class="Special"> &lt;- </span>next-duplex *before-cursor
+  op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+  *op<span class="Special"> &lt;- </span>merge <span class="Constant">0/insert-operation</span>, cursor-row-before, cursor-column-before, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, insert-from, insert-to, <span class="Constant">0/never-coalesce</span>
+  editor<span class="Special"> &lt;- </span>add-operation editor, op
+]
+
+<span class="Comment"># Everytime you add a new operation to the undo stack, be sure to clear the</span>
+<span class="Comment"># redo stack, because it's now obsolete.</span>
+<span class="Comment"># Beware: since we're counting cursor moves as operations, this means just</span>
+<span class="Comment"># moving the cursor can lose work on the undo stack.</span>
+<span class="muRecipe">recipe</span> add-operation [
+  <span class="Constant">local-scope</span>
+  editor:address:editor-data<span class="Special"> &lt;- </span><span class="Constant">next-ingredient</span>
+  op:address:operation<span class="Special"> &lt;- </span><span class="Constant">next-ingredient</span>
+  undo:address:address:list:address:operation<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+  *undo<span class="Special"> &lt;- </span>push op *undo
+  redo:address:address:list:address:operation<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">redo:offset</span>
+  *redo<span class="Special"> &lt;- </span>copy <span class="Constant">0</span>
+  <span class="muControl">reply</span> editor/same-as-ingredient:<span class="Constant">0</span>
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-undo&gt;</span> [
+  <span class="Delimiter">{</span>
+    typing:address:insert-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">typing:variant</span>
+    <span class="muControl">break-unless</span> typing
+    start:address:duplex-list<span class="Special"> &lt;- </span>get *typing, <span class="Constant">insert-from:offset</span>
+    end:address:duplex-list<span class="Special"> &lt;- </span>get *typing, <span class="Constant">insert-until:offset</span>
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    *before-cursor<span class="Special"> &lt;- </span>prev-duplex start
+    remove-duplex-between *before-cursor, end
+    *cursor-row<span class="Special"> &lt;- </span>get *typing, <span class="Constant">before-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *typing, <span class="Constant">before-column:offset</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    *top<span class="Special"> &lt;- </span>get *typing, <span class="Constant">before-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-typing-multiple [
+  <span class="Comment"># create an editor and type multiple characters</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[012]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># all characters must be gone</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-typing-multiple-2 [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># type some characters</span>
+  assume-console [
+    type <span class="Constant">[012]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .012a      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># back to original text</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[3]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .3a        .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-typing-enter [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[  abc]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># new line</span>
+  assume-console [
+    left-click <span class="Constant">1</span>, <span class="Constant">8</span>
+    press enter
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  abc     .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># line is indented</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">5</span>
+  ]
+  <span class="Comment"># back to original text</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  abc     .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># cursor should be at end of line</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  abc1    .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="Comment"># redo typing</span>
+
+<span class="muScenario">scenario</span> editor-redo-typing [
+  <span class="Comment"># create an editor, type something, undo</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[012]</span>
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># all characters must be back</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .012a      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[3]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .0123a     .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-redo&gt;</span> [
+  <span class="Delimiter">{</span>
+    typing:address:insert-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">typing:variant</span>
+    <span class="muControl">break-unless</span> typing
+    insert-from:address:duplex-list<span class="Special"> &lt;- </span>get *typing, <span class="Constant">insert-from:offset</span>  <span class="Comment"># ignore insert-to because it's already been spliced away</span>
+    <span class="Comment"># assert insert-to matches next-duplex(*before-cursor)</span>
+    insert-duplex-range *before-cursor, insert-from
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    *cursor-row<span class="Special"> &lt;- </span>get *typing, <span class="Constant">after-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *typing, <span class="Constant">after-column:offset</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    *top<span class="Special"> &lt;- </span>get *typing, <span class="Constant">after-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="muScenario">scenario</span> editor-redo-typing-empty [
+  <span class="Comment"># create an editor, type something, undo</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[012]</span>
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># all characters must be back</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .012       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[3]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .0123      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-work-clears-redo-stack [
+  <span class="Comment"># create an editor with some text, do some work, undo</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[1]</span>
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># do some more work</span>
+  assume-console [
+    type <span class="Constant">[0]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .0abc      .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># nothing should happen</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .0abc      .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-redo-typing-and-enter-and-tab [
+  <span class="Comment"># create an editor</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># insert some text and tabs, hit enter, some more text and tabs</span>
+  assume-console [
+    press tab
+    type <span class="Constant">[ab]</span>
+    press tab
+    type <span class="Constant">[cd]</span>
+    press enter
+    press tab
+    type <span class="Constant">[efg]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+   <span class="Constant"> .    efg   .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">7</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># typing in second line deleted, but not indent</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># undo again</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># indent and newline deleted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">8</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># undo again</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># empty screen</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">8</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo again</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># newline and indent inserted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo again</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># indent and newline deleted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">7</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .  ab  cd  .</span>
+   <span class="Constant"> .    efg   .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="Comment"># undo cursor movement and scroll</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-touch [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor</span>
+  assume-console [
+    left-click <span class="Constant">3</span>, <span class="Constant">1</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># click undone</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .1abc      .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;move-cursor-begin&gt;</span> [
+  before-cursor-row:number<span class="Special"> &lt;- </span>get *editor, <span class="Constant">cursor-row:offset</span>
+  before-cursor-column:number<span class="Special"> &lt;- </span>get *editor, <span class="Constant">cursor-column:offset</span>
+  before-top-of-screen:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;move-cursor-end&gt;</span> [
+  after-cursor-row:number<span class="Special"> &lt;- </span>get *editor, <span class="Constant">cursor-row:offset</span>
+  after-cursor-column:number<span class="Special"> &lt;- </span>get *editor, <span class="Constant">cursor-column:offset</span>
+  after-top-of-screen:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+  <span class="Delimiter">{</span>
+    <span class="muControl">break-unless</span> undo-coalesce-tag
+    <span class="Comment"># if previous operation was also a move, and also had the same coalesce</span>
+    <span class="Comment"># tag, coalesce with it</span>
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    <span class="muControl">break-unless</span> *undo
+    op:address:operation<span class="Special"> &lt;- </span>first *undo
+    move:address:move-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">move:variant</span>
+    <span class="muControl">break-unless</span> move
+    previous-coalesce-tag:number<span class="Special"> &lt;- </span>get *move, <span class="Constant">tag:offset</span>
+    coalesce?:boolean<span class="Special"> &lt;- </span>equal undo-coalesce-tag, previous-coalesce-tag
+    <span class="muControl">break-unless</span> coalesce?
+    after-row:address:number<span class="Special"> &lt;- </span>get-address *move, <span class="Constant">after-row:offset</span>
+    *after-row<span class="Special"> &lt;- </span>copy after-cursor-row
+    after-column:address:number<span class="Special"> &lt;- </span>get-address *move, <span class="Constant">after-column:offset</span>
+    *after-column<span class="Special"> &lt;- </span>copy after-cursor-column
+    after-top:address:number<span class="Special"> &lt;- </span>get-address *move, <span class="Constant">after-top-of-screen:offset</span>
+    *after-top<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    <span class="muControl">break</span> <span class="Constant">+done-adding-move-operation:label</span>
+  <span class="Delimiter">}</span>
+  op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+  *op<span class="Special"> &lt;- </span>merge <span class="Constant">1/move-operation</span>, before-cursor-row, before-cursor-column, before-top-of-screen, after-cursor-row, after-cursor-column, after-top-of-screen, undo-coalesce-tag
+  editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">  +done-adding-move-operation</span>
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-undo&gt;</span> [
+  <span class="Delimiter">{</span>
+    move:address:move-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">move:variant</span>
+    <span class="muControl">break-unless</span> move
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">top-of-screen:offset</span>
+    *cursor-row<span class="Special"> &lt;- </span>get *move, <span class="Constant">before-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *move, <span class="Constant">before-column:offset</span>
+    *top<span class="Special"> &lt;- </span>get *move, <span class="Constant">before-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-scroll [
+  <span class="Comment"># screen has 1 line for menu + 3 lines</span>
+  assume-screen <span class="Constant">5/width</span>, <span class="Constant">4/height</span>
+  <span class="Comment"># editor contains a wrapped line</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a</span>
+<span class="Constant">b</span>
+<span class="Constant">cdefgh]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">5/right</span>
+  <span class="Comment"># position cursor at end of screen and try to move right</span>
+  assume-console [
+    left-click <span class="Constant">3</span>, <span class="Constant">3</span>
+    press right-arrow
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  <span class="Comment"># screen scrolls</span>
+  screen-should-contain [
+   <span class="Constant"> .     .</span>
+   <span class="Constant"> .b    .</span>
+<span class="Constant">    .cdef↩.</span>
+   <span class="Constant"> .gh   .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moved back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  <span class="Comment"># scroll undone</span>
+  screen-should-contain [
+   <span class="Constant"> .     .</span>
+   <span class="Constant"> .a    .</span>
+   <span class="Constant"> .b    .</span>
+<span class="Constant">    .cdef↩.</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .     .</span>
+   <span class="Constant"> .b    .</span>
+<span class="Constant">    .cde1↩.</span>
+   <span class="Constant"> .fgh  .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-left-arrow [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor</span>
+  assume-console [
+    left-click <span class="Constant">3</span>, <span class="Constant">1</span>
+    press left-arrow
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .g1hi      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-up-arrow [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor</span>
+  assume-console [
+    left-click <span class="Constant">3</span>, <span class="Constant">1</span>
+    press up-arrow
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .g1hi      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-down-arrow [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press down-arrow
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .d1ef      .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-ctrl-f [
+  <span class="Comment"># create an editor with multiple pages of text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a</span>
+<span class="Constant">b</span>
+<span class="Constant">c</span>
+<span class="Constant">d</span>
+<span class="Constant">e</span>
+<span class="Constant">f]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># scroll the page</span>
+  assume-console [
+    press ctrl-f
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># screen should again show page 1</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+   <span class="Constant"> .b         .</span>
+   <span class="Constant"> .c         .</span>
+   <span class="Constant"> .d         .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-page-down [
+  <span class="Comment"># create an editor with multiple pages of text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a</span>
+<span class="Constant">b</span>
+<span class="Constant">c</span>
+<span class="Constant">d</span>
+<span class="Constant">e</span>
+<span class="Constant">f]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># scroll the page</span>
+  assume-console [
+    press page-down
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># screen should again show page 1</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+   <span class="Constant"> .b         .</span>
+   <span class="Constant"> .c         .</span>
+   <span class="Constant"> .d         .</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-ctrl-b [
+  <span class="Comment"># create an editor with multiple pages of text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a</span>
+<span class="Constant">b</span>
+<span class="Constant">c</span>
+<span class="Constant">d</span>
+<span class="Constant">e</span>
+<span class="Constant">f]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># scroll the page down and up</span>
+  assume-console [
+    press page-down
+    press ctrl-b
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># screen should again show page 2</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .d         .</span>
+   <span class="Constant"> .e         .</span>
+   <span class="Constant"> .f         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-page-up [
+  <span class="Comment"># create an editor with multiple pages of text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[a</span>
+<span class="Constant">b</span>
+<span class="Constant">c</span>
+<span class="Constant">d</span>
+<span class="Constant">e</span>
+<span class="Constant">f]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># scroll the page down and up</span>
+  assume-console [
+    press page-down
+    press page-up
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># screen should again show page 2</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .d         .</span>
+   <span class="Constant"> .e         .</span>
+   <span class="Constant"> .f         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-ctrl-a [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor, then to start of line</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press ctrl-a
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .d1ef      .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-home [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor, then to start of line</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press home
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .d1ef      .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-ctrl-e [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor, then to start of line</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press ctrl-e
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .d1ef      .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-end [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor, then to start of line</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press end
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves back</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .d1ef      .</span>
+   <span class="Constant"> .ghi       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-separates-undo-insert-from-undo-cursor-move [
+  <span class="Comment"># create an editor, type some text, move the cursor, type some more text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    type <span class="Constant">[abc]</span>
+    left-click <span class="Constant">1</span>, <span class="Constant">1</span>
+    type <span class="Constant">[d]</span>
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .adbc      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># last letter typed is deleted</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># undo again</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># no change to screen; cursor moves</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  <span class="Comment"># undo again</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># screen empty</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .          .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># first insert</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  <span class="Comment"># redo again</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># redo again</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># second insert</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .adbc      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+]
+
+<span class="muScenario">scenario</span> editor-can-undo-multiple-arrows-in-the-same-direction [
+  <span class="Comment"># create an editor with some text</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># move the cursor</span>
+  assume-console [
+    left-click <span class="Constant">2</span>, <span class="Constant">1</span>
+    press right-arrow
+    press right-arrow
+    press up-arrow
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># up-arrow is undone</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  <span class="Comment"># undo again</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># both right-arrows are undone</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+]
+
+<span class="Comment"># redo cursor movement and scroll</span>
+
+<span class="muScenario">scenario</span> editor-redo-touch [
+  <span class="Comment"># create an editor with some text, click on a character, undo</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def</span>
+<span class="Constant">ghi]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  assume-console [
+    left-click <span class="Constant">3</span>, <span class="Constant">1</span>
+    press ctrl-z
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+    <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+    <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  ]
+  <span class="Comment"># cursor moves to left-click</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .def       .</span>
+   <span class="Constant"> .g1hi      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-redo&gt;</span> [
+  <span class="Delimiter">{</span>
+    move:address:move-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">move:variant</span>
+    <span class="muControl">break-unless</span> move
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    *cursor-row<span class="Special"> &lt;- </span>get *move, <span class="Constant">after-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *move, <span class="Constant">after-column:offset</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    *top<span class="Special"> &lt;- </span>get *move, <span class="Constant">after-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># undo backspace</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-and-redo-backspace [
+  <span class="Comment"># create an editor</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># insert some text and hit backspace</span>
+  assume-console [
+    type <span class="Constant">[abc]</span>
+    press backspace
+    press backspace
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="Comment"># save operation to undo</span>
+<span class="muRecipe">after</span> <span class="Constant">&lt;backspace-character-begin&gt;</span> [
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;backspace-character-end&gt;</span> [
+  <span class="Delimiter">{</span>
+    <span class="muControl">break-unless</span> backspaced-cell  <span class="Comment"># backspace failed; don't add an undo operation</span>
+    top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    <span class="Delimiter">{</span>
+      <span class="Comment"># if previous operation was an insert, coalesce this operation with it</span>
+      <span class="muControl">break-unless</span> *undo
+      op:address:operation<span class="Special"> &lt;- </span>first *undo
+      deletion:address:delete-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">delete:variant</span>
+      <span class="muControl">break-unless</span> deletion
+      previous-coalesce-tag:number<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">tag:offset</span>
+      coalesce?:boolean<span class="Special"> &lt;- </span>equal previous-coalesce-tag, <span class="Constant">1/coalesce-backspace</span>
+      <span class="muControl">break-unless</span> coalesce?
+      delete-from:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">delete-from:offset</span>
+      *delete-from<span class="Special"> &lt;- </span>copy *before-cursor
+      backspaced-so-far:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">deleted-text:offset</span>
+      insert-duplex-range backspaced-cell, *backspaced-so-far
+      *backspaced-so-far<span class="Special"> &lt;- </span>copy backspaced-cell
+      after-row:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-row:offset</span>
+      *after-row<span class="Special"> &lt;- </span>copy *cursor-row
+      after-column:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-column:offset</span>
+      *after-column<span class="Special"> &lt;- </span>copy *cursor-column
+      after-top:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-top-of-screen:offset</span>
+      *after-top<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+      <span class="muControl">break</span> <span class="Constant">+done-adding-backspace-operation:label</span>
+    <span class="Delimiter">}</span>
+    <span class="Comment"># if not, create a new operation</span>
+    op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+    deleted-until:address:duplex-list<span class="Special"> &lt;- </span>next-duplex *before-cursor
+    *op<span class="Special"> &lt;- </span>merge <span class="Constant">2/delete-operation</span>, save-row/<span class="muRecipe">before</span>, save-column/<span class="muRecipe">before</span>, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, backspaced-cell/deleted, *before-cursor/delete-from, deleted-until, <span class="Constant">1/coalesce-backspace</span>
+    editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">    +done-adding-backspace-operation</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-undo&gt;</span> [
+  <span class="Delimiter">{</span>
+    deletion:address:delete-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">delete:variant</span>
+    <span class="muControl">break-unless</span> deletion
+    start2:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">data:offset</span>
+    anchor:address:duplex-list<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">delete-from:offset</span>
+    <span class="muControl">break-unless</span> anchor
+    deleted:address:duplex-list<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">deleted-text:offset</span>
+    old-cursor:address:duplex-list<span class="Special"> &lt;- </span>last-duplex deleted
+    insert-duplex-range anchor, deleted
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    *before-cursor<span class="Special"> &lt;- </span>copy old-cursor
+    *cursor-row<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">before-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">before-column:offset</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    *top<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">before-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;handle-redo&gt;</span> [
+  <span class="Delimiter">{</span>
+    deletion:address:delete-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">delete:variant</span>
+    <span class="muControl">break-unless</span> deletion
+    start:address:duplex-list<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">delete-from:offset</span>
+    end:address:duplex-list<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">delete-until:offset</span>
+    remove-duplex-between start, end
+    <span class="Comment"># assert cursor-row/cursor-column/top-of-screen match after-row/after-column/after-top-of-screen</span>
+    *cursor-row<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">after-row:offset</span>
+    *cursor-column<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">after-column:offset</span>
+    top:address:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    *top<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">after-top-of-screen:offset</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># undo delete</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-and-redo-delete [
+  <span class="Comment"># create an editor</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># insert some text and hit delete and backspace a few times</span>
+  assume-console [
+    type <span class="Constant">[abcdef]</span>
+    left-click <span class="Constant">1</span>, <span class="Constant">2</span>
+    press delete
+    press backspace
+    press delete
+    press delete
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .af        .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># undo deletes</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .adef      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># undo backspace</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abdef     .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># undo first delete</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abcdef    .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo first delete</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abdef     .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo backspace</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .adef      .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Comment"># redo deletes</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .af        .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;delete-character-begin&gt;</span> [
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;delete-character-end&gt;</span> [
+  <span class="Delimiter">{</span>
+    <span class="muControl">break-unless</span> deleted-cell  <span class="Comment"># delete failed; don't add an undo operation</span>
+    top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    <span class="Delimiter">{</span>
+      <span class="Comment"># if previous operation was an insert, coalesce this operation with it</span>
+      <span class="muControl">break-unless</span> *undo
+      op:address:operation<span class="Special"> &lt;- </span>first *undo
+      deletion:address:delete-operation<span class="Special"> &lt;- </span>maybe-convert *op, <span class="Constant">delete:variant</span>
+      <span class="muControl">break-unless</span> deletion
+      previous-coalesce-tag:number<span class="Special"> &lt;- </span>get *deletion, <span class="Constant">tag:offset</span>
+      coalesce?:boolean<span class="Special"> &lt;- </span>equal previous-coalesce-tag, <span class="Constant">2/coalesce-delete</span>
+      <span class="muControl">break-unless</span> coalesce?
+      delete-until:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">delete-until:offset</span>
+      *delete-until<span class="Special"> &lt;- </span>next-duplex *before-cursor
+      deleted-so-far:address:address:duplex-list<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">deleted-text:offset</span>
+      *deleted-so-far<span class="Special"> &lt;- </span>append-duplex *deleted-so-far, deleted-cell
+      after-row:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-row:offset</span>
+      *after-row<span class="Special"> &lt;- </span>copy *cursor-row
+      after-column:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-column:offset</span>
+      *after-column<span class="Special"> &lt;- </span>copy *cursor-column
+      after-top:address:number<span class="Special"> &lt;- </span>get-address *deletion, <span class="Constant">after-top-of-screen:offset</span>
+      *after-top<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+      <span class="muControl">break</span> <span class="Constant">+done-adding-delete-operation:label</span>
+    <span class="Delimiter">}</span>
+    <span class="Comment"># if not, create a new operation</span>
+    op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+    deleted-until:address:duplex-list<span class="Special"> &lt;- </span>next-duplex *before-cursor
+    *op<span class="Special"> &lt;- </span>merge <span class="Constant">2/delete-operation</span>, save-row/<span class="muRecipe">before</span>, save-column/<span class="muRecipe">before</span>, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, deleted-cell/deleted, *before-cursor/delete-from, deleted-until, <span class="Constant">2/coalesce-delete</span>
+    editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">    +done-adding-delete-operation</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># undo ctrl-k</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-and-redo-ctrl-k [
+  <span class="Comment"># create an editor</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># insert some text and hit delete and backspace a few times</span>
+  assume-console [
+    left-click <span class="Constant">1</span>, <span class="Constant">1</span>
+    press ctrl-k
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a         .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .a1        .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;delete-to-end-of-line-begin&gt;</span> [
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;delete-to-end-of-line-end&gt;</span> [
+  <span class="Delimiter">{</span>
+    <span class="muControl">break-unless</span> deleted-cells  <span class="Comment"># delete failed; don't add an undo operation</span>
+    top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+    deleted-until:address:duplex-list<span class="Special"> &lt;- </span>next-duplex *before-cursor
+    *op<span class="Special"> &lt;- </span>merge <span class="Constant">2/delete-operation</span>, save-row/<span class="muRecipe">before</span>, save-column/<span class="muRecipe">before</span>, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, deleted-cells/deleted, *before-cursor/delete-from, deleted-until, <span class="Constant">0/never-coalesce</span>
+    editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">    +done-adding-delete-operation</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># undo ctrl-u</span>
+
+<span class="muScenario">scenario</span> editor-can-undo-and-redo-ctrl-u [
+  <span class="Comment"># create an editor</span>
+  assume-screen <span class="Constant">10/width</span>, <span class="Constant">5/height</span>
+  <span class="Constant">1</span>:address:array:character<span class="Special"> &lt;- </span>new <span class="Constant">[abc</span>
+<span class="Constant">def]</span>
+  <span class="Constant">2</span>:address:editor-data<span class="Special"> &lt;- </span>new-editor <span class="Constant">1</span>:address:array:character, screen:address, <span class="Constant">0/left</span>, <span class="Constant">10/right</span>
+  editor-render screen, <span class="Constant">2</span>:address:editor-data
+  <span class="Comment"># insert some text and hit delete and backspace a few times</span>
+  assume-console [
+    left-click <span class="Constant">1</span>, <span class="Constant">2</span>
+    press ctrl-u
+  ]
+  editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .c         .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  <span class="Comment"># undo</span>
+  assume-console [
+    press ctrl-z
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .abc       .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">2</span>
+  ]
+  <span class="Comment"># redo</span>
+  assume-console [
+    press ctrl-y
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  <span class="Comment"># first line inserted</span>
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .c         .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-row:offset</span>
+  <span class="Constant">4</span>:number<span class="Special"> &lt;- </span>get *<span class="Constant">2</span>:address:editor-data, <span class="Constant">cursor-column:offset</span>
+  memory-should-contain [
+    <span class="Constant">3</span><span class="Special"> &lt;- </span><span class="Constant">1</span>
+    <span class="Constant">4</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
+  ]
+  <span class="Comment"># cursor should be in the right place</span>
+  assume-console [
+    type <span class="Constant">[1]</span>
+  ]
+  run [
+    editor-event-loop screen:address, console:address, <span class="Constant">2</span>:address:editor-data
+  ]
+  screen-should-contain [
+   <span class="Constant"> .          .</span>
+   <span class="Constant"> .1c        .</span>
+   <span class="Constant"> .def       .</span>
+<span class="Constant">    .┈┈┈┈┈┈┈┈┈┈.</span>
+   <span class="Constant"> .          .</span>
+  ]
+]
+
+<span class="muRecipe">after</span> <span class="Constant">&lt;delete-to-start-of-line-begin&gt;</span> [
+  top-before:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+]
+<span class="muRecipe">before</span> <span class="Constant">&lt;delete-to-start-of-line-end&gt;</span> [
+  <span class="Delimiter">{</span>
+    <span class="muControl">break-unless</span> deleted-cells  <span class="Comment"># delete failed; don't add an undo operation</span>
+    top-after:address:duplex-list<span class="Special"> &lt;- </span>get *editor, <span class="Constant">top-of-screen:offset</span>
+    undo:address:address:list<span class="Special"> &lt;- </span>get-address *editor, <span class="Constant">undo:offset</span>
+    op:address:operation<span class="Special"> &lt;- </span>new <span class="Constant">operation:type</span>
+    deleted-until:address:duplex-list<span class="Special"> &lt;- </span>next-duplex *before-cursor
+    *op<span class="Special"> &lt;- </span>merge <span class="Constant">2/delete-operation</span>, save-row/<span class="muRecipe">before</span>, save-column/<span class="muRecipe">before</span>, top-before, *cursor-row/<span class="muRecipe">after</span>, *cursor-column/<span class="muRecipe">after</span>, top-after, deleted-cells/deleted, *before-cursor/delete-from, deleted-until, <span class="Constant">0/never-coalesce</span>
+    editor<span class="Special"> &lt;- </span>add-operation editor, op
+<span class="Constant">    +done-adding-delete-operation</span>
+  <span class="Delimiter">}</span>
+]
+
+<span class="Comment"># todo:</span>
+<span class="Comment"># operations for recipe side and each sandbox-data</span>
+<span class="Comment"># undo delete sandbox as a separate primitive on the status bar</span>
+</pre>
+</body>
+</html>
+<!-- vim: set foldmethod=manual : -->
author	Kartik K. Agaram <vc@akkartik.com>	2015-09-07 10:37:27 -0700
committer	Kartik K. Agaram <vc@akkartik.com>	2015-09-07 10:37:27 -0700
commit	f5465e1220d73e237c51897b7d1211ec53b0dc04 (patch)
tree	939ee8e57241b8515aede8106c6420e330ace75a /html/edit/011-editor-undo.mu.html
parent	5ccf2653fb7d31b013f77df4e92e964e45c54f8a (diff)
download	mu-f5465e1220d73e237c51897b7d1211ec53b0dc04.tar.gz