sclass VMBusListenerWithLivelinessCheck implements IF2<S, O, O> {
  Set listeners; // where we were added
  IF0<Bool> livelinessCheck;
  O listener;
  bool verbose;
  
  *(Set *listeners, IF0<Bool> *livelinessCheck, O *listener) {}
  *(Set *listeners, IF0<Bool> *livelinessCheck, IVF2<S, O> *listener) {}
  
  public O get(S msg, O arg) {
    bool live = false;
    pcall {
      live = livelinessCheck == null || livelinessCheck.get();
    }
    if (!live) {
      if (verbose)
        print VMBusListenerWithLivelinessCheck("Removing VM bus listener");
      remove(listeners, this);
      null;
    }
    ret callF(listener, msg, arg);
  }
}

Began life as a copy of #1031569