sclass WaitForAnswer_YesNo {
  swappable S answer(S s) {
    if (isYes(s)) {
      cancel();
      ret processAnswer(true);
    }
    if "no" {
      cancel();
      ret processAnswer(false);
    }
    ret otherInput(s);
  }
  
  swappable bool isYes(S s) { ret main isYes(s); }
  swappable bool isNo(S s) { ret main isNo(s); }
  swappable S otherInput(S s) { ret "I don't understand. Yes or no?"; }
  swappable S processAnswer(bool yes) { ret "You said " + yesNo_short(yes); }
  swappable void cancel() { print("Cancelling"); }
}