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