}

  @Override
  @CheckForNull
  public Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  @CheckForNull
  public Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {
    Entry<K, V> entry = firstEntry();

  }

  @Override
  @CheckForNull
  public Entry<K, V> pollFirstEntry() {
    return Iterators.pollNext(entryIterator());
  }

  @Override
  @CheckForNull
  public Entry<K, V> pollLastEntry() {
    return Iterators.pollNext(descendingEntryIterator());
  }

  @Override
  @ParametricNullness
  public K firstKey() {
    Entry<K, V> entry = firstEntry();

   * override {@link #iterator} you may wish to override {@link #pollFirst} to forward to this
   * implementation.
   */
  @CheckForNull
  protected E standardPollFirst() {
    return Iterators.pollNext(iterator());
  }

  @Override
  @CheckForNull
  public E pollLast() {
    return delegate().pollLast();
  }

  /**

    };
  }

  /**
   * Deletes and returns the next value from the iterator, or returns {@code null} if there is no
   * such value.
   */
  @CheckForNull
  static <T extends @Nullable Object> T pollNext(Iterator<T> iterator) {
    if (iterator.hasNext()) {
      T result = iterator.next();
      iterator.remove();
      return result;
    } else {
      return null;
    }
  }