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,

   * iterating over it or any of its {@code descendingSet}, {@code subSet}, {@code headSet}, or
   * {@code tailSet} views.
   *
   * <pre>{@code
   * NavigableSet<E> set = synchronizedNavigableSet(new TreeSet<E>());
   *  ...
   * synchronized (set) {
   *   // Must be in the synchronized block
   *   Iterator<E> it = set.iterator();
   *   while (it.hasNext()) {
   *     foo(it.next());

    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

    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