anchor.diff:  
   /// Converts the top stack element between the specified types. Instead of
   /// reinterpreting the memory, the value is retained. For example, executing a
  -/// type conversion `double32↦int32` on the value `1.0` yields `1` (which has
  +/// type conversion `double32↦uint32` on the value `1.0` yields `1` (which has
   /// a different bit pattern).
  +///
  +/// The VM instruction set only supports the following subset of all to/from
  +/// conversion pairs:
  +///
  +/// integer width expansion – only conversion between adjacent sizes.
  +///
  +///     uint8↦uint16
  +///     uint16↦uint32
  +///     uint32↦uint64
  +///     sint8↦sint16
  +///     sint16↦sint32
  +///     sint32↦sint64
  +///
  +/// integer width reduction – these simply crop the most significant bytes,
  +/// which yields identical behaviour for signed and unsigned values.
  +///
  +///     uint16↦uint8
  +///     uint32↦uint16
  +///     uint64↦uint32
  +///
  +/// int / float conversion – only between same-size types.
  +///
  +///     uint32↦float
  +///     sint32↦float
  +///     float↦uint32
  +///     float↦sint32
  +///     uint64↦double
  +///     sint64↦double
  +///     double↦uint64
  +///     double↦sint64
  +///
  +/// float width
  +///
  +///     float↦double
  +///     double↦float
   class TypeConversionInstruction extends Instruction {
     final NumberType from;
     final NumberType to;
  
positive.code:         ..forEach((instruction) =>
              instruction.opcode = 1 + instructions.indexOf(instruction)))([
      new PushInstruction(NumberType.uint8),
      new PushInstruction(NumberType.uint16)
    ]);
  
    /// The program that is executed when calling [run].
  
negative.code: /// methods for it. Instead, [AddInstruction] is instantiated for each number
  /// type.
  abstract class Instruction {
    /// A verbose mnemonic for this instruction. If two instructions have the same
    /// name, they yield the same result if executed on a VM.
    String get name;
  
    /// This number of bytes immediately following this instruction are passed to
  