NavigableMap<K, V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> map) {
    checkNotNull(map);
    if (map instanceof UnmodifiableNavigableMap) {
      @SuppressWarnings("unchecked") // covariant
      NavigableMap<K, V> result = (NavigableMap<K, V>) map;
      return result;
    } else {
      return new UnmodifiableNavigableMap<>(map);
    }
  }

  @CheckForNull