@CheckForNull transient NavigableSet<E> descendingSet;

    @Override
    public NavigableSet<E> descendingSet() {
      synchronized (mutex) {
        if (descendingSet == null) {
          NavigableSet<E> dS = Synchronized.navigableSet(delegate().descendingSet(), mutex);
          descendingSet = dS;
          return dS;
        }
        return descendingSet;
      }
    }

    @Override

    @Override
    protected SortedSet<String> create(String[] elements) {
      return ImmutableSortedSet.<String>reverseOrder().add(elements).build().descendingSet();
    }
  }

  public static class ImmutableSortedSetExplicitComparator extends TestStringSetGenerator {

    private static final Comparator<String> STRING_REVERSED = Collections.reverseOrder();

  }

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

  @Override
  public NavigableSet<E> descendingSet() {
    return new SafeTreeSet<>(delegate.descendingSet());
  }

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

  @Override
  public @Nullable E floor(E e) {

    return forward.firstEntry();
  }

  @Override
  public int size() {
    return forward.size();
  }

  @Override
  public ImmutableSortedSet<E> elementSet() {
    return forward.elementSet().descendingSet();
  }

  @Override
  Entry<E> getEntry(int index) {
    return forward.entrySet().asList().reverse().get(index);
  }

  @Override
  public ImmutableSortedMultiset<E> descendingMultiset() {

    return forward.firstEntry();
  }

  @Override
  public int size() {
    return forward.size();
  }

  @Override
  public ImmutableSortedSet<E> elementSet() {
    return forward.elementSet().descendingSet();
  }

  @Override
  Entry<E> getEntry(int index) {
    return forward.entrySet().asList().reverse().get(index);
  }

  @Override
  public ImmutableSortedMultiset<E> descendingMultiset() {

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

        assertThat(filtered.descendingSet())
            .containsExactlyElementsIn(unfiltered.descendingSet())
            .inOrder();
      }
    }

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

  @CheckForNull
  transient ImmutableSortedSet<E> descendingSet;

  /** @since 12.0 */
  @GwtIncompatible // NavigableSet
  @Override
  public ImmutableSortedSet<E> descendingSet() {
    // racy single-check idiom
    ImmutableSortedSet<E> result = descendingSet;
    if (result == null) {
      result = descendingSet = createDescendingSet();
      result.descendingSet = this;
    }
    return result;
  }

      return new WrappedNavigableSet(key, wrapped, (getAncestor() == null) ? this : getAncestor());
    }

    @Override
    public NavigableSet<V> descendingSet() {
      return wrap(getSortedSetDelegate().descendingSet());
    }

    @Override
    public Iterator<V> descendingIterator() {
      return new WrappedIterator(getSortedSetDelegate().descendingIterator());
    }

    @Override

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

        assertThat(filtered.descendingSet())
            .containsExactlyElementsIn(unfiltered.descendingSet())
            .inOrder();
      }
    }

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

  @CheckForNull
  transient ImmutableSortedSet<E> descendingSet;

  /** @since 12.0 */
  @GwtIncompatible // NavigableSet
  @Override
  public ImmutableSortedSet<E> descendingSet() {
    // racy single-check idiom
    ImmutableSortedSet<E> result = descendingSet;
    if (result == null) {
      result = descendingSet = createDescendingSet();
      result.descendingSet = this;
    }
    return result;
  }