code: boolean enqueue(Reference<? extends T> reference) {
          synchronized (lock) {
              if (enqueueLocked(reference)) {
                  lock.notifyAll();
                  return true;
              }
              return false;
          }
      }
docs: Enqueue the reference object on the receiver.
  
  @param reference
  reference object to be enqueued.
  @return true if the reference was enqueued.