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) {