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diff --git a/034exclusive_container.cc b/034exclusive_container.cc
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+++ b/034exclusive_container.cc
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+//: Exclusive containers contain exactly one of a fixed number of 'variants'
+//: of different types.
+//:
+//: They also implicitly contain a tag describing precisely which variant is
+//: currently stored in them.
+
+:(before "End Mu Types Initialization")
+//: We'll use this container as a running example, with two number elements.
+{
+type_ordinal tmp = put(Type_ordinal, "number-or-point", Next_type_ordinal++);
+get_or_insert(Type, tmp);  // initialize
+get(Type, tmp).kind = EXCLUSIVE_CONTAINER;
+get(Type, tmp).name = "number-or-point";
+get(Type, tmp).elements.push_back(reagent("i:number"));
+get(Type, tmp).elements.push_back(reagent("p:point"));
+}
+
+//: Tests in this layer often explicitly set up memory before reading it as a
+//: container. Don't do this in general. I'm tagging such cases with /unsafe;
+//: they'll be exceptions to later checks.
+:(scenario copy_exclusive_container)
+# Copying exclusive containers copies all their contents and an extra location for the tag.
+def main [
+  1:num <- copy 1  # 'point' variant
+  2:num <- copy 34
+  3:num <- copy 35
+  4:number-or-point <- copy 1:number-or-point/unsafe
+]
++mem: storing 1 in location 4
++mem: storing 34 in location 5
++mem: storing 35 in location 6
+
+:(before "End size_of(type) Special-cases")
+if (t.kind == EXCLUSIVE_CONTAINER) {
+  // size of an exclusive container is the size of its largest variant
+  // (So like containers, it can't contain arrays.)
+  int result = 0;
+  for (int i = 0; i < SIZE(t.elements); ++i) {
+    reagent tmp;
+    tmp.type = new type_tree(*type);
+    int size = size_of(variant_type(tmp, i));
+    if (size > result) result = size;
+  }
+  // ...+1 for its tag.
+  return result+1;
+}
+
+//:: To access variants of an exclusive container, use 'maybe-convert'.
+//: It always returns an address (so that you can modify it) or null (to
+//: signal that the conversion failed (because the container contains a
+//: different variant).
+
+//: 'maybe-convert' requires a literal in ingredient 1. We'll use a synonym
+//: called 'variant'.
+:(before "End Mu Types Initialization")
+put(Type_ordinal, "variant", 0);
+
+:(scenario maybe_convert)
+def main [
+  12:num <- copy 1
+  13:num <- copy 35
+  14:num <- copy 36
+  20:point, 22:bool <- maybe-convert 12:number-or-point/unsafe, 1:variant
+]
+# boolean
++mem: storing 1 in location 22
+# point
++mem: storing 35 in location 20
++mem: storing 36 in location 21
+
+:(scenario maybe_convert_fail)
+def main [
+  12:num <- copy 1
+  13:num <- copy 35
+  14:num <- copy 36
+  20:num, 21:bool <- maybe-convert 12:number-or-point/unsafe, 0:variant
+]
+# boolean
++mem: storing 0 in location 21
+# number: no write
+
+:(before "End Primitive Recipe Declarations")
+MAYBE_CONVERT,
+:(before "End Primitive Recipe Numbers")
+put(Recipe_ordinal, "maybe-convert", MAYBE_CONVERT);
+:(before "End Primitive Recipe Checks")
+case MAYBE_CONVERT: {
+  const recipe& caller = get(Recipe, r);
+  if (SIZE(inst.ingredients) != 2) {
+    raise << maybe(caller.name) << "'maybe-convert' expects exactly 2 ingredients in '" << to_original_string(inst) << "'\n" << end();
+    break;
+  }
+  reagent/*copy*/ base = inst.ingredients.at(0);
+  // Update MAYBE_CONVERT base in Check
+  if (!base.type) {
+    raise << maybe(caller.name) << "first ingredient of 'maybe-convert' should be an exclusive-container, but got '" << base.original_string << "'\n" << end();
+    break;
+  }
+  const type_tree* base_type = base.type;
+  // Update MAYBE_CONVERT base_type in Check
+  if (!base_type->atom || base_type->value == 0 || !contains_key(Type, base_type->value) || get(Type, base_type->value).kind != EXCLUSIVE_CONTAINER) {
+    raise << maybe(caller.name) << "first ingredient of 'maybe-convert' should be an exclusive-container, but got '" << base.original_string << "'\n" << end();
+    break;
+  }
+  if (!is_literal(inst.ingredients.at(1))) {
+    raise << maybe(caller.name) << "second ingredient of 'maybe-convert' should have type 'variant', but got '" << inst.ingredients.at(1).original_string << "'\n" << end();
+    break;
+  }
+  if (inst.products.empty()) break;
+  if (SIZE(inst.products) != 2) {
+    raise << maybe(caller.name) << "'maybe-convert' expects exactly 2 products in '" << to_original_string(inst) << "'\n" << end();
+    break;
+  }
+  reagent/*copy*/ product = inst.products.at(0);
+  // Update MAYBE_CONVERT product in Check
+  reagent& offset = inst.ingredients.at(1);
+  populate_value(offset);
+  if (offset.value >= SIZE(get(Type, base_type->value).elements)) {
+    raise << maybe(caller.name) << "invalid tag " << offset.value << " in '" << to_original_string(inst) << '\n' << end();
+    break;
+  }
+  const reagent& variant = variant_type(base, offset.value);
+  if (!types_coercible(product, variant)) {
+    raise << maybe(caller.name) << "'maybe-convert " << base.original_string << ", " << inst.ingredients.at(1).original_string << "' should write to " << to_string(variant.type) << " but '" << product.name << "' has type " << to_string(product.type) << '\n' << end();
+    break;
+  }
+  reagent/*copy*/ status = inst.products.at(1);
+  // Update MAYBE_CONVERT status in Check
+  if (!is_mu_boolean(status)) {
+    raise << maybe(get(Recipe, r).name) << "second product yielded by 'maybe-convert' should be a boolean, but tried to write to '" << inst.products.at(1).original_string << "'\n" << end();
+    break;
+  }
+  break;
+}
+:(before "End Primitive Recipe Implementations")
+case MAYBE_CONVERT: {
+  reagent/*copy*/ base = current_instruction().ingredients.at(0);
+  // Update MAYBE_CONVERT base in Run
+  int base_address = base.value;
+  if (base_address == 0) {
+    raise << maybe(current_recipe_name()) << "tried to access location 0 in '" << to_original_string(current_instruction()) << "'\n" << end();
+    break;
+  }
+  int tag = current_instruction().ingredients.at(1).value;
+  reagent/*copy*/ product = current_instruction().products.at(0);
+  // Update MAYBE_CONVERT product in Run
+  reagent/*copy*/ status = current_instruction().products.at(1);
+  // Update MAYBE_CONVERT status in Run
+  // optimization: directly write results to only update first product when necessary
+  write_products = false;
+  if (tag == static_cast<int>(get_or_insert(Memory, base_address))) {
+    const reagent& variant = variant_type(base, tag);
+    trace("mem") << "storing 1 in location " << status.value << end();
+    put(Memory, status.value, 1);
+    if (!is_dummy(product)) {
+      // Write Memory in Successful MAYBE_CONVERT in Run
+      for (int i = 0;  i < size_of(variant);  ++i) {
+        double val = get_or_insert(Memory, base_address+/*skip tag*/1+i);
+        trace("mem") << "storing " << no_scientific(val) << " in location " << product.value+i << end();
+        put(Memory, product.value+i, val);
+      }
+    }
+  }
+  else {
+    trace("mem") << "storing 0 in location " << status.value << end();
+    put(Memory, status.value, 0);
+  }
+  break;
+}
+
+:(code)
+const reagent variant_type(const reagent& base, int tag) {
+  return variant_type(base.type, tag);
+}
+
+const reagent variant_type(const type_tree* type, int tag) {
+  assert(tag >= 0);
+  const type_tree* root_type = type->atom ? type : type->left;
+  assert(contains_key(Type, root_type->value));
+  assert(!get(Type, root_type->value).name.empty());
+  const type_info& info = get(Type, root_type->value);
+  assert(info.kind == EXCLUSIVE_CONTAINER);
+  reagent/*copy*/ element = info.elements.at(tag);
+  // End variant_type Special-cases
+  return element;
+}
+
+:(scenario maybe_convert_product_type_mismatch)
+% Hide_errors = true;
+def main [
+  12:num <- copy 1
+  13:num <- copy 35
+  14:num <- copy 36
+  20:num, 21:bool <- maybe-convert 12:number-or-point/unsafe, 1:variant
+]
++error: main: 'maybe-convert 12:number-or-point/unsafe, 1:variant' should write to point but '20' has type number
+
+:(scenario maybe_convert_dummy_product)
+def main [
+  12:num <- copy 1
+  13:num <- copy 35
+  14:num <- copy 36
+  _, 21:bool <- maybe-convert 12:number-or-point/unsafe, 1:variant
+]
+$error: 0
+
+//:: Allow exclusive containers to be defined in Mu code.
+
+:(scenario exclusive_container)
+exclusive-container foo [
+  x:num
+  y:num
+]
++parse: --- defining exclusive-container foo
++parse: element: {x: "number"}
++parse: element: {y: "number"}
+
+:(before "End Command Handlers")
+else if (command == "exclusive-container") {
+  insert_container(command, EXCLUSIVE_CONTAINER, in);
+}
+
+//: arrays are disallowed inside exclusive containers unless their length is
+//: fixed in advance
+
+:(scenario exclusive_container_contains_array)
+exclusive-container foo [
+  x:array:num:3
+]
+$error: 0
+
+:(scenario exclusive_container_disallows_dynamic_array_element)
+% Hide_errors = true;
+exclusive-container foo [
+  x:array:num
+]
++error: container 'foo' cannot determine size of element 'x'
+
+//:: To construct exclusive containers out of variant types, use 'merge'.
+:(scenario lift_to_exclusive_container)
+exclusive-container foo [
+  x:num
+  y:num
+]
+def main [
+  1:num <- copy 34
+  2:foo <- merge 0/x, 1:num  # tag must be a literal when merging exclusive containers
+  4:foo <- merge 1/y, 1:num
+]
++mem: storing 0 in location 2
++mem: storing 34 in location 3
++mem: storing 1 in location 4
++mem: storing 34 in location 5
+
+//: type-checking for 'merge' on exclusive containers
+
+:(scenario merge_handles_exclusive_container)
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  z:num
+]
+def main [
+  1:foo <- merge 0/x, 34
+]
++mem: storing 0 in location 1
++mem: storing 34 in location 2
+$error: 0
+
+:(scenario merge_requires_literal_tag_for_exclusive_container)
+% Hide_errors = true;
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  z:num
+]
+def main [
+  1:num <- copy 0
+  2:foo <- merge 1:num, 34
+]
++error: main: ingredient 0 of 'merge' should be a literal, for the tag of exclusive-container 'foo' in '2:foo <- merge 1:num, 34'
+
+:(scenario merge_handles_exclusive_container_inside_exclusive_container)
+exclusive-container foo [
+  x:num
+  y:bar
+]
+exclusive-container bar [
+  a:num
+  b:num
+]
+def main [
+  1:num <- copy 0
+  2:bar <- merge 0/a, 34
+  4:foo <- merge 1/y, 2:bar
+]
++mem: storing 0 in location 5
++mem: storing 34 in location 6
+$error: 0
+
+:(before "End check_merge_call Special-cases")
+case EXCLUSIVE_CONTAINER: {
+  assert(state.data.top().container_element_index == 0);
+  trace("transform") << "checking exclusive container " << to_string(container) << " vs ingredient " << ingredient_index << end();
+  // easy case: exact match
+  if (types_strictly_match(container, inst.ingredients.at(ingredient_index)))
+    return;
+  if (!is_literal(ingredients.at(ingredient_index))) {
+    raise << maybe(caller.name) << "ingredient " << ingredient_index << " of 'merge' should be a literal, for the tag of exclusive-container '" << container_info.name << "' in '" << to_original_string(inst) << "'\n" << end();
+    return;
+  }
+  reagent/*copy*/ ingredient = ingredients.at(ingredient_index);  // unnecessary copy just to keep this function from modifying caller
+  populate_value(ingredient);
+  if (ingredient.value >= SIZE(container_info.elements)) {
+    raise << maybe(caller.name) << "invalid tag at " << ingredient_index << " for '" << container_info.name << "' in '" << to_original_string(inst) << "'\n" << end();
+    return;
+  }
+  const reagent& variant = variant_type(container, ingredient.value);
+  trace("transform") << "tag: " << ingredient.value << end();
+  // replace union with its variant
+  state.data.pop();
+  state.data.push(merge_check_point(variant, 0));
+  ++ingredient_index;
+  break;
+}
+
+:(scenario merge_check_container_containing_exclusive_container)
+container foo [
+  x:num
+  y:bar
+]
+exclusive-container bar [
+  x:num
+  y:num
+]
+def main [
+  1:foo <- merge 23, 1/y, 34
+]
++mem: storing 23 in location 1
++mem: storing 1 in location 2
++mem: storing 34 in location 3
+$error: 0
+
+:(scenario merge_check_container_containing_exclusive_container_2)
+% Hide_errors = true;
+container foo [
+  x:num
+  y:bar
+]
+exclusive-container bar [
+  x:num
+  y:num
+]
+def main [
+  1:foo <- merge 23, 1/y, 34, 35
+]
++error: main: too many ingredients in '1:foo <- merge 23, 1/y, 34, 35'
+
+:(scenario merge_check_exclusive_container_containing_container)
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  x:num
+  y:num
+]
+def main [
+  1:foo <- merge 1/y, 23, 34
+]
++mem: storing 1 in location 1
++mem: storing 23 in location 2
++mem: storing 34 in location 3
+$error: 0
+
+:(scenario merge_check_exclusive_container_containing_container_2)
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  x:num
+  y:num
+]
+def main [
+  1:foo <- merge 0/x, 23
+]
+$error: 0
+
+:(scenario merge_check_exclusive_container_containing_container_3)
+% Hide_errors = true;
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  x:num
+  y:num
+]
+def main [
+  1:foo <- merge 1/y, 23
+]
++error: main: too few ingredients in '1:foo <- merge 1/y, 23'
+
+:(scenario merge_check_exclusive_container_containing_container_4)
+exclusive-container foo [
+  x:num
+  y:bar
+]
+container bar [
+  a:num
+  b:num
+]
+def main [
+  1:bar <- merge 23, 24
+  3:foo <- merge 1/y, 1:bar
+]
+$error: 0
+
+//: Since the different variants of an exclusive-container might have
+//: different sizes, relax the size mismatch check for 'merge' instructions.
+:(before "End size_mismatch(x) Special-cases")
+if (current_step_index() < SIZE(Current_routine->steps())
+    && current_instruction().operation == MERGE
+    && !current_instruction().products.empty()
+    && current_instruction().products.at(0).type) {
+  reagent/*copy*/ x = current_instruction().products.at(0);
+  // Update size_mismatch Check for MERGE(x)
+  const type_tree* root_type = x.type->atom ? x.type : x.type->left;
+  assert(root_type->atom);
+  if (get(Type, root_type->value).kind == EXCLUSIVE_CONTAINER)
+    return size_of(x) < SIZE(data);
+}
+
+:(scenario merge_exclusive_container_with_mismatched_sizes)
+container foo [
+  x:num
+  y:num
+]
+exclusive-container bar [
+  x:num
+  y:foo
+]
+def main [
+  1:num <- copy 34
+  2:num <- copy 35
+  3:bar <- merge 0/x, 1:num
+  6:bar <- merge 1/foo, 1:num, 2:num
+]
++mem: storing 0 in location 3
++mem: storing 34 in location 4
+# bar is always 3 large so location 5 is skipped
++mem: storing 1 in location 6
++mem: storing 34 in location 7
++mem: storing 35 in location 8