}

    @Override
    public Iterator<E> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public @Nullable E pollFirst() {
      return getElementOrNull(multiset().pollFirstEntry());
    }

    @Override
    public @Nullable E pollLast() {
      return getElementOrNull(multiset().pollLastEntry());
    }

        NavigableSet<Integer> filtered = filter(createUnfiltered(contents), EVEN);
        NavigableSet<Integer> unfiltered = createUnfiltered(filtered);

        assertEquals(unfiltered.pollFirst(), filtered.pollFirst());
        assertEquals(unfiltered, filtered);
      }
    }

    public void testPollLast() {
      for (List<Integer> contents : SAMPLE_INPUTS) {

      return standardCeiling(e);
    }

    @Override
    public @Nullable T higher(T e) {
      return standardHigher(e);
    }

    @Override
    public @Nullable T pollFirst() {
      return standardPollFirst();
    }

    @Override
    public @Nullable T pollLast() {
      return standardPollLast();
    }

    @Override

    public @Nullable E lower(E e) {
      synchronized (mutex) {
        return delegate().lower(e);
      }
    }

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

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

    public @Nullable E lower(E e) {
      synchronized (mutex) {
        return delegate().lower(e);
      }
    }

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

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

    public @Nullable E higher(@ParametricNullness E e) {
      return forward.lower(e);
    }

    @Override
    public @Nullable E pollFirst() {
      return forward.pollLast();
    }

    @Override
    public @Nullable E pollLast() {
      return forward.pollFirst();
    }

    @Override
    public NavigableSet<E> descendingSet() {
      return forward;
    }

    @Override

    }

    @Override
    public @Nullable V higher(@ParametricNullness V v) {
      return getSortedSetDelegate().higher(v);
    }

    @Override
    public @Nullable V pollFirst() {
      return Iterators.pollNext(iterator());
    }

    @Override
    public @Nullable V pollLast() {
      return Iterators.pollNext(descendingIterator());
    }

    }
  }

  /**
   * Removes and returns the least element of this queue, or returns {@code null} if the queue is
   * empty.
   */
  @CanIgnoreReturnValue
  public @Nullable E pollFirst() {
    return poll();
  }

  /**
   * Removes and returns the least element of this queue.
   *
   * @throws NoSuchElementException if the queue is empty
   */
  @CanIgnoreReturnValue

      return map().ceilingKey(e);
    }

    @Override
    public @Nullable K higher(@ParametricNullness K e) {
      return map().higherKey(e);
    }

    @Override
    public @Nullable K pollFirst() {
      return keyOrNull(map().pollFirstEntry());
    }

    @Override
    public @Nullable K pollLast() {
      return keyOrNull(map().pollLastEntry());
    }

    @Override

      return map().ceilingKey(e);
    }

    @Override
    public @Nullable K higher(@ParametricNullness K e) {
      return map().higherKey(e);
    }

    @Override
    public @Nullable K pollFirst() {
      return keyOrNull(map().pollFirstEntry());
    }

    @Override
    public @Nullable K pollLast() {
      return keyOrNull(map().pollLastEntry());
    }

    @Override