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