// synchronized
sclass SeenAndQueue<A> {
  Set<A> seen = syncSet();
  L<A> queue = syncLinkedList();
  
  transient L onAddedToQueue = syncList(); // L<Runnable|voidfunc(A)>
  transient L<Runnable> onQueueEmpty = syncList();

  // queue handler  
  transient ReliableSingleThread rst;
  
  *() {}
  *(Runnable handler) { rst(handler); }
  
  bool add(A a) {
    if (a == null || !seen.add(a)) false;
    queue.add(a);
    pcallFAll(onAddedToQueue, a);
    true;
  }
  
  void addAll(Iterable<A> l) { fOr (A a : l) add(a); }
  
  A popQueue() {
    ping();
    synchronized(queue) {
      A a = syncPopFirst(queue);
      if (a != null && empty(queue))
        pcallFAll(onQueueEmpty);
      ret a;
    }
  }
  
  // ends when queue is empty
  ItIt<A> queueIterator() {
    ret iteratorFromFunction_if0(() -> popQueue());
  }
  
  void onAddedToQueue(O r) { onAddedToQueue.add(r); }
  
  ReliableSingleThread rst(Runnable handler) {
    if (rst != null) fail("rst already set");
    rst = ReliableSingleThread(handler);
    onAddedToQueue.add(rst);
    ret rst;
  }
  
  void cleanMeUp {
    cleanUp(rst);
    rst = null;
  }
}