@CanIgnoreReturnValue // pushed down from class to method
  @Override
  public @Nullable E poll(long timeout, TimeUnit unit) throws InterruptedException {
    final Monitor monitor = this.monitor;
    if (monitor.enterWhen(notEmpty, timeout, unit)) {
      try {
        return q.poll();
      } finally {
        monitor.leave();
      }
    } else {
      return null;
    }
  }