sclass AbstractSayAndInputBot {
  VF1<S> say; // primary output listener
  L onOutput; // more output listeners (L<VF1<S>>)
  F0<S> input;
  
  void say(S s) {
    callF(say, s);
    pcallFAll(onOutput, s);
  }
  
  void say(Iterable<S> l) {
    for unnull (S s : l) say(s);
  }
  
  S input() {
    ret callF(input);
  }
}