* If you override {@link #descendingIterator} you may wish to override {@link #pollLast} to
   * forward to this implementation.
   */
  protected @Nullable E standardPollLast() {
    return Iterators.pollNext(descendingIterator());
  }

  @ParametricNullness
  protected E standardFirst() {
    return iterator().next();
  }

  @ParametricNullness

   * If you override {@link #descendingIterator} you may wish to override {@link #pollLast} to
   * forward to this implementation.
   */
  protected @Nullable E standardPollLast() {
    return Iterators.pollNext(descendingIterator());
  }

  @ParametricNullness
  protected E standardFirst() {
    return iterator().next();
  }

  @ParametricNullness