}

  public void testDescendingSet() {
    NavigableSet<String> set = create();
    NavigableSet<String> descendingSet = set.descendingSet();
    assertTrue(descendingSet instanceof SynchronizedNavigableSet);
    assertSame(set, ((SynchronizedNavigableSet<String>) descendingSet).mutex);
  }

  public void testHeadSet_E() {
    NavigableSet<String> set = create();
    SortedSet<String> headSet = set.headSet("a");

  }

  public void testDescendingSet() {
    NavigableSet<String> map = create();
    NavigableSet<String> descendingSet = map.descendingSet();
    assertTrue(descendingSet instanceof SynchronizedNavigableSet);
    assertSame(MUTEX, ((SynchronizedNavigableSet<String>) descendingSet).mutex);
  }

  public void testHeadSet_E() {
    NavigableSet<String> map = create();
    SortedSet<String> headSet = map.headSet("a");

    NavigableMap<String, Integer> map = create();
    NavigableSet<String> descendingKeySet = map.descendingKeySet();
    assertTrue(descendingKeySet instanceof SynchronizedNavigableSet);
    assertSame(mutex, ((SynchronizedNavigableSet<String>) descendingKeySet).mutex);
  }

  public void testDescendingMap() {
    NavigableMap<String, Integer> map = create();

    NavigableMap<String, Integer> map = create();
    NavigableSet<String> descendingKeySet = map.descendingKeySet();
    assertTrue(descendingKeySet instanceof SynchronizedNavigableSet);
    assertSame(mutex, ((SynchronizedNavigableSet<String>) descendingKeySet).mutex);
  }

  public void testDescendingMap() {
    NavigableMap<String, Integer> map = create();

                NavigableSet<String> delegate = new TreeSet<>(MinimalCollection.of(elements));
                return Collections.synchronizedNavigableSet(delegate);
              }
            })
        .named("synchronizedNavigableSet/TreeSet, natural")
        .withFeatures(
            SetFeature.GENERAL_PURPOSE,
            CollectionFeature.SERIALIZABLE,

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible // NavigableSet
  @VisibleForTesting
  static final class SynchronizedNavigableSet<E extends @Nullable Object>
      extends SynchronizedSortedSet<E> implements NavigableSet<E> {
    SynchronizedNavigableSet(NavigableSet<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

    @Override

   *
   * @param navigableSet the navigable set to be "wrapped" in a synchronized navigable set.
   * @return a synchronized view of the specified navigable set.
   * @since 13.0
   */
  @GwtIncompatible // NavigableSet
  public static <E extends @Nullable Object> NavigableSet<E> synchronizedNavigableSet(
      NavigableSet<E> navigableSet) {