anchor.diff:  
     /// The `i31` heap type.
     static const i31 = I31HeapType._();
  -
  -  /// The `extern` heap type.
  -  static const extern = ExternHeapType._();
   
     /// Whether this heap type is nullable by default, i.e. when written with the
     /// -`ref` shorthand. A `null` value here means the heap type has no default
  
positive.code:   bool isSubtypeOf(HeapType other) => other == HeapType.any;
  
    @override
    void serialize(Serializer s) => s.writeByte(0x6E);
  
    @override
    String toString() => "any";
  
negative.code:   const RefType.i31({bool nullable = I31HeapType.defaultNullability})
        : this._(HeapType.i31, nullable);
  
    /// A (possibly nullable) reference to a custom heap type.
    RefType.def(DefType defType, {required bool nullable})
        : this(defType, nullable: nullable);
  