svoid runInQAndWait(Q q, Runnable r) {
  if (r == null) ret;
  if (isInQ(q)) ret with callF(r);
  
  final new Flag done;
  final new Var<Throwable> error;
  
  q.add(r {
    try {
      callF(r);
    } catch e {
      error.set(e);
    } finally {
      done.raise();
    }
  });
  
  done.waitUntilUp();
  if (error.has()) throw rethrow(error!);
}