sclass Either<A, B> {
  byte which;
  O value;
  
  *() {}
  *(int which, O *value) { this.which = (byte) which; }
  
  bool isA() { ret which == 1; }
  bool isB() { ret which == 2; }
  
  A a() { if (which != 1) _failMe(); ret (A) value; }
  B b() { if (which != 2) _failMe(); ret (B) value; }
  
  A aOpt() { ret which != 1 ? null : (A) value; }
  B bOpt() { ret which != 2 ? null : (B) value; }
  
  void _failMe {
    fail("Either object is of wrong type: " + shortClassName(value));
  }
  
  toString {
    ret "Either" + (isA() ? "A" : "B") + "(" + value + ")";
  }
}

Began life as a copy of #1009167