code: public E poll() {
          final ReentrantLock lock = this.lock;
          lock.lock();
          try {
              E first = q.peek();
              return (first == null || first.getDelay(NANOSECONDS) > 0)
                  ? null
                  : q.poll();
          } finally {
              lock.unlock();
          }
      }
docs: Retrieves and removes the head of this queue, or returns {@code null}
  if this queue has no elements with an expired delay.
  
  @return the head of this queue, or {@code null} if this
  queue has no elements with an expired delay