static AutoCloseable dm_vmBus_answerToMessage(S msg, final F0 f) {
  final DynModule m = dm_current_mandatory();
  Map<S, Set> map = vm_busListenersByMessage_live();
  AutoCloseable c;
  synchronized(map) {
    Set listeners = map.get(msg);
    if (listeners == null) map.put(msg, listeners = syncIdentityHashSet());
    // We're technically violating the one-synchronized-object-per-thread rule,
    // but it should be OK here.
    // TODO: remove empty sets from map
    c = tempAdd(listeners, func(S _msg, O arg) {
      temp m.enter();
      ret callF(f, arg);
    });
  }
  dm_ownResource(c);
  ret c;
}