static <A> O mapMethodLike vmBus_onMessage_weakRef(S msg, IVF1<A> onMessage) {
  Map<S, Set> map = vm_busListenersByMessage_live();
  WeakRefVMBusListener listener = new((IVF2<S, O>) (_msg, arg) -> onMessage.get(arg));
  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
    listeners.add(listener);
  }
  ret listener;
}