}

  @GwtIncompatible // NavigableMap
  private static final class UnmodifiableNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingSortedMap<K, V> implements NavigableMap<K, V>, Serializable {
    private final NavigableMap<K, ? extends V> delegate;

    UnmodifiableNavigableMap(NavigableMap<K, ? extends V> delegate) {
      this.delegate = delegate;