code: public final boolean offer(Queue<E> queue, E item) throws ConcurrentConveyorException {
          if (queue.offer(item)) {
              return true;
          } else {
              checkDrainerGone();
              unparkDrainer();
              return false;
          }
      }
docs: Offers an item to the given queue. No check is performed that the queue
  actually belongs to this conveyor.
  
  @return whether the item was accepted by the queue
  @throws ConcurrentConveyorException if the draining thread has already left