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diff --git a/033exclusive_container.cc b/033exclusive_container.cc
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+++ b/033exclusive_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_number tmp = Type_number["number-or-point"] = Next_type_number++;
+Type[tmp].size = 2;
+Type[tmp].kind = exclusive_container;
+Type[tmp].name = "number-or-point";
+//? cout << tmp << ": " << SIZE(Type[tmp].elements) << '\n'; //? 1
+vector<type_number> t1;
+t1.push_back(number);
+Type[tmp].elements.push_back(t1);
+//? cout << SIZE(Type[tmp].elements) << '\n'; //? 1
+vector<type_number> t2;
+t2.push_back(point);
+Type[tmp].elements.push_back(t2);
+//? cout << SIZE(Type[tmp].elements) << '\n'; //? 1
+//? cout << "point: " << point << '\n'; //? 1
+Type[tmp].element_names.push_back("i");
+Type[tmp].element_names.push_back("p");
+}
+
+//: Tests in this layer often explicitly setup memory before reading it as an
+//: array. Don't do this in general. I'm tagging exceptions with /raw to
+//: avoid warnings.
+:(scenario copy_exclusive_container)
+# Copying exclusive containers copies all their contents and an extra location for the tag.
+recipe main [
+  1:number <- copy 1:literal  # 'point' variant
+  2:number <- copy 34:literal
+  3:number <- copy 35:literal
+  4:number-or-point <- copy 1:number-or-point/raw  # unsafe
+]
++mem: storing 1 in location 4
++mem: storing 34 in location 5
++mem: storing 35 in location 6
+
+:(before "End size_of(types) 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.)
+//?   cout << "--- " << types.at(0) << ' ' << t.size << '\n'; //? 1
+//?   cout << "point: " << Type_number["point"] << " " << Type[Type_number["point"]].name << " " << Type[Type_number["point"]].size << '\n'; //? 1
+//?   cout << t.name << ' ' << t.size << ' ' << SIZE(t.elements) << '\n'; //? 1
+  long long int result = 0;
+  for (long long int i = 0; i < t.size; ++i) {
+    long long int tmp = size_of(t.elements.at(i));
+//?     cout << i << ": " << t.elements.at(i).at(0) << ' ' << tmp << ' ' << result << '\n'; //? 1
+    if (tmp > result) result = tmp;
+  }
+  // ...+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")
+Type_number["variant"] = 0;
+
+:(scenario maybe_convert)
+recipe main [
+  12:number <- copy 1:literal
+  13:number <- copy 35:literal
+  14:number <- copy 36:literal
+  20:address:point <- maybe-convert 12:number-or-point/raw, 1:variant  # unsafe
+]
++mem: storing 13 in location 20
+
+:(scenario maybe_convert_fail)
+recipe main [
+  12:number <- copy 1:literal
+  13:number <- copy 35:literal
+  14:number <- copy 36:literal
+  20:address:point <- maybe-convert 12:number-or-point/raw, 0:variant  # unsafe
+]
++mem: storing 0 in location 20
+
+:(before "End Primitive Recipe Declarations")
+MAYBE_CONVERT,
+:(before "End Primitive Recipe Numbers")
+Recipe_number["maybe-convert"] = MAYBE_CONVERT;
+:(before "End Primitive Recipe Implementations")
+case MAYBE_CONVERT: {
+  reagent base = canonize(current_instruction().ingredients.at(0));
+  long long int base_address = base.value;
+  type_number base_type = base.types.at(0);
+  assert(Type[base_type].kind == exclusive_container);
+  assert(is_literal(current_instruction().ingredients.at(1)));
+  long long int tag = current_instruction().ingredients.at(1).value;
+  long long int result;
+  if (tag == static_cast<long long int>(Memory[base_address])) {
+    result = base_address+1;
+  }
+  else {
+    result = 0;
+  }
+  products.resize(1);
+  products.at(0).push_back(result);
+  break;
+}
+
+//:: Allow exclusive containers to be defined in mu code.
+
+:(scenario exclusive_container)
+exclusive-container foo [
+  x:number
+  y:number
+]
++parse: reading exclusive-container foo
++parse:   element name: x
++parse:   type: 1
++parse:   element name: y
++parse:   type: 1
+
+:(before "End Command Handlers")
+else if (command == "exclusive-container") {
+  insert_container(command, exclusive_container, in);
+}
+
+//:: To construct exclusive containers out of variant types, use 'merge'.
+:(scenario lift_to_exclusive_container)
+exclusive-container foo [
+  x:number
+  y:number
+]
+
+recipe main [
+  1:number <- copy 34:literal
+  2:foo <- merge 0:literal/x, 1:number
+  4:foo <- merge 1:literal/x, 1:number
+]
++mem: storing 0 in location 2
++mem: storing 34 in location 3
++mem: storing 1 in location 4
++mem: storing 34 in location 5