static class YN<Y, N> {
  Y yes;  // invariant: !sYes => (yes == null)
  N no;   // invariant: isYes => (no == null)
  bool isYes;
  
  *() {}
  *(bool *isYes) {}
  *(bool *isYes, Y *yes) {}
  *(bool *isYes, Y *yes, N *no) {}
}