if (isEmpty()) {
            return null;
        }
        return header.previous;
    }

    /**
     * 最初の要素を削除します。
     *
     * @return 最初の要素
     */
    public E removeFirst() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        final E first = header.next.element;
        header.next.remove();
        return first;
    }

    /**

        } finally {
          if (doReadTimeout) {
            readTimeout.exitAndThrowIfTimedOut()
          }
        }
      }
      if (headersQueue.isNotEmpty()) {
        return headersQueue.removeFirst()
      }
      throw errorException ?: StreamResetException(errorCode!!)
    }
  }

  /**
   * Returns the trailers. It is only safe to call this once the source stream has been completely

    return poll();
  }

  /**
   * Removes and returns the least element of this queue.
   *
   * @throws NoSuchElementException if the queue is empty
   */
  @CanIgnoreReturnValue
  public E removeFirst() {
    return remove();
  }

  /**
   * Retrieves, but does not remove, the least element of this queue, or returns {@code null} if the
   * queue is empty.
   */
  @CheckForNull

    public boolean offerLast(E e) {
      synchronized (mutex) {
        return delegate().offerLast(e);
      }
    }

    @Override
    public E removeFirst() {
      synchronized (mutex) {
        return delegate().removeFirst();
      }
    }

    @Override
    public E removeLast() {
      synchronized (mutex) {
        return delegate().removeLast();
      }
    }

    public boolean offerLast(E e) {
      synchronized (mutex) {
        return delegate().offerLast(e);
      }
    }

    @Override
    public E removeFirst() {
      synchronized (mutex) {
        return delegate().removeFirst();
      }
    }

    @Override
    public E removeLast() {
      synchronized (mutex) {
        return delegate().removeLast();
      }
    }

    return poll();
  }

  /**
   * Removes and returns the least element of this queue.
   *
   * @throws NoSuchElementException if the queue is empty
   */
  @CanIgnoreReturnValue
  public E removeFirst() {
    return remove();
  }

  /**
   * Retrieves, but does not remove, the least element of this queue, or returns {@code null} if the
   * queue is empty.
   */
  @CheckForNull

      while (topMetaIterator == null || !topMetaIterator.hasNext()) {
        if (metaIterators != null && !metaIterators.isEmpty()) {
          topMetaIterator = metaIterators.removeFirst();
        } else {
          return null;
        }
      }
      return topMetaIterator;
    }

    @Override
    public boolean hasNext() {

    }

    @Override
    public V remove(final Object key) {
        final Entry<K, V> e = removeMap(key);
        if (e != null) {
            final V value = e.value;
            removeList(entryIndexOf(e));
            e.clear();
            return value;
        }
        return null;
    }

    /**
     * インデックスで指定された位置のエントリを削除します。
     *
     * @param index