static S callAnswerMethod(O responder, S s) {
  ret callAnswerMethod(responder, s, litlist(s));
}

static S callAnswerMethod(O responder, S s, L<S> history) {
  S answer = (S) callOpt(responder, "answer", s, history);
  if (answer == null)
    answer = (S) callOpt(responder, "answer", s);
  return emptyToNull(answer);
}