return new SynchronizedNavigableMap<>(navigableMap, mutex);
  }

  @GwtIncompatible // NavigableMap
  @VisibleForTesting
  static final class SynchronizedNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedSortedMap<K, V> implements NavigableMap<K, V> {

    SynchronizedNavigableMap(NavigableMap<K, V> delegate, @Nullable Object mutex) {

    return new SynchronizedNavigableMap<>(navigableMap, mutex);
  }

  @GwtIncompatible // NavigableMap
  @VisibleForTesting
  static final class SynchronizedNavigableMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedSortedMap<K, V> implements NavigableMap<K, V> {

    SynchronizedNavigableMap(NavigableMap<K, V> delegate, @Nullable Object mutex) {