@Override
    public E pop() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pop();
    }

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

    private static final long serialVersionUID = 0;
  }

  @SuppressWarnings("CheckReturnValue")
  public void testHoldsLockOnAllOperations() {

    return forward.contains(object);
  }

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

  @Override
  public UnmodifiableIterator<E> iterator() {
    return forward.descendingIterator();
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {
    return forward.tailSet(toElement, inclusive).descendingSet();
  }

  @Override

  }

  @Override
  public boolean containsAll(Collection<?> c) {
    return delegate.containsAll(c);
  }

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

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

  @Override
  public E first() {

    return forward.contains(object);
  }

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

  @Override
  public UnmodifiableIterator<E> iterator() {
    return forward.descendingIterator();
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {
    return forward.tailSet(toElement, inclusive).descendingSet();
  }

  @Override

    }

    @Override
    public Iterator<E> descendingIterator() {
      return Iterators.filter(unfiltered().descendingIterator(), predicate);
    }

    @Override
    @ParametricNullness
    public E last() {
      return Iterators.find(unfiltered().descendingIterator(), predicate);
    }

    @Override
    public NavigableSet<E> subSet(

  @Override
  @CheckForNull
  public E lower(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, false).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E floor(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, true).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {

        return equalsOrThrow(previous, last) ? null : domain.next(previous);
      }
    };
  }

  @GwtIncompatible // NavigableSet
  @Override
  public UnmodifiableIterator<C> descendingIterator() {
    return new AbstractSequentialIterator<C>(last()) {
      final C first = first();

      @Override
      @CheckForNull
      protected C computeNext(C previous) {

  }

  @CollectionSize.Require(SEVERAL)
  public void testDescendingNavigation() {
    List<E> descending = new ArrayList<>();
    for (Iterator<E> i = navigableSet.descendingIterator(); i.hasNext(); ) {
      descending.add(i.next());
    }
    Collections.reverse(descending);
    assertEquals(values, descending);
  }

  public void testEmptySubSet() {

  }

  @CollectionSize.Require(SEVERAL)
  public void testDescendingNavigation() {
    List<E> descending = new ArrayList<>();
    for (Iterator<E> i = navigableSet.descendingIterator(); i.hasNext(); ) {
      descending.add(i.next());
    }
    Collections.reverse(descending);
    assertEquals(values, descending);
  }

  public void testEmptySubSet() {

  @Override
  @CheckForNull
  public E lower(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, false).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E floor(E e) {
    return Iterators.<@Nullable E>getNext(headSet(e, true).descendingIterator(), null);
  }

  /** @since 12.0 */
  @Override
  @CheckForNull
  public E ceiling(E e) {