sclass Alt<A, B> {
  byte which;
  O value;
  
  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; }
  
  void _failMe {
    fail("Alt object is of wrong type: " + shortClassName(value));
  }
}