// synchronized
sclass SeenAndQueue<A> {
  Set<A> seen = syncSet();
  L<A> queue = syncLinkedList();
  
  // L<Runnable|voidfunc(A)>
  transient L onAddedToQueue = syncList();
  
  transient L<Runnable> onQueueEmpty = syncList();
  
  bool add(A a) {
    if (!seen.add(a)) false;
    queue.add(a);
    pcallFAll(onAddedToQueue, a);
    true;
  }
  
  A popQueue() {
    ping();
    synchronized(queue) {
      A a = syncPopFirst(queue);
      if (a != null && empty(queue))
        pcallFAll(onQueueEmpty);
      ret a;
    }
  }
  
  void onAddedToQueue(O r) { onAddedToQueue.add(r); }
}