}

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(
      NavigableSet<E> navigableSet, @CheckForNull Object mutex) {
    return new SynchronizedNavigableSet<>(navigableSet, mutex);
  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(NavigableSet<E> navigableSet) {
    return navigableSet(navigableSet, null);

  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(
      NavigableSet<E> navigableSet, @CheckForNull Object mutex) {
    return new SynchronizedNavigableSet<>(navigableSet, mutex);
  }

  @GwtIncompatible // NavigableSet
  static <E extends @Nullable Object> NavigableSet<E> navigableSet(NavigableSet<E> navigableSet) {
    return navigableSet(navigableSet, null);

    }
  }

  @GwtIncompatible // NavigableSet
  private static class FilteredNavigableSet<E extends @Nullable Object> extends FilteredSortedSet<E>
      implements NavigableSet<E> {
    FilteredNavigableSet(NavigableSet<E> unfiltered, Predicate<? super E> predicate) {
      super(unfiltered, predicate);
    }

    NavigableSet<E> unfiltered() {
      return (NavigableSet<E>) unfiltered;
    }

    @Override

    }

    @Override
    <E extends @Nullable Object> Collection<E> unmodifiableCollectionSubclass(
        Collection<E> collection) {
      if (collection instanceof NavigableSet) {
        return Sets.unmodifiableNavigableSet((NavigableSet<E>) collection);
      } else if (collection instanceof SortedSet) {
        return Collections.unmodifiableSortedSet((SortedSet<E>) collection);
      } else if (collection instanceof Set) {