code: @GwtIncompatible // NavigableSet
    public static <E> NavigableSet<E> unmodifiableNavigableSet(NavigableSet<E> set) {
      if (set instanceof ImmutableSortedSet || set instanceof UnmodifiableNavigableSet) {
        return set;
      }
      return new UnmodifiableNavigableSet<E>(set);
    }
docs: Returns an unmodifiable view of the specified navigable set. This method
  allows modules to provide users with "read-only" access to internal
  navigable sets. Query operations on the returned set "read through" to the
  specified set, and attempts to modify the returned set, whether direct or
  via its collection views, result in an
  {@code UnsupportedOperationException}.
  
  <p>The returned navigable set will be serializable if the specified
  navigable set is serializable.
  
  @param set the navigable set for which an unmodifiable view is to be
  returned
  @return an unmodifiable view of the specified navigable set
  @since 12.0