sclass WeakRefVMBusListener extends WeakReference implements IF2<S, O> {
  Set listeners; // where we were added
  
  *(S *msg, O listener) { super(listener); }
  
  O get(S msg, O arg) {
    O listener = get();
    if (listener == null) {
      listeners.remove(listener);
      null;
    }
    ret callF(listener, msg, arg);
  }
}