code: @GwtIncompatible("NavigableMap")
    public static <K, V> NavigableMap<K, V> unmodifiableNavigableMap(NavigableMap<K, V> map) {
      checkNotNull(map);
      if (map instanceof UnmodifiableNavigableMap) {
        return map;
      } else {
        return new UnmodifiableNavigableMap<K, V>(map);
      }
    }
docs: Returns an unmodifiable view of the specified navigable map. Query operations on the returned
  map read through to the specified map, and attempts to modify the returned map, whether direct
  or via its views, result in an {@code UnsupportedOperationException}.
  
  <p>The returned navigable map will be serializable if the specified navigable map is
  serializable.
  
  @param map the navigable map for which an unmodifiable view is to be returned
  @return an unmodifiable view of the specified navigable map
  @since 12.0