* forward to this implementation.
   */
  protected @Nullable E standardPollLast() {
    return Iterators.pollNext(descendingIterator());
  }

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

  @ParametricNullness
  protected E standardLast() {
    return descendingIterator().next();
  }

  @Override
  public NavigableSet<E> descendingSet() {