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) {

   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * {@snippet :
   * NavigableMap<K, V> map = synchronizedNavigableMap(new TreeMap<K, V>());
   *
   * // Needn't be in synchronized block
   * NavigableSet<K> set = map.navigableKeySet();
   *
   * synchronized (map) { // Synchronizing on map, not set!

   * descendingMap}, {@code subMap}, {@code headMap} or {@code tailMap} views.
   *
   * {@snippet :
   * NavigableMap<K, V> map = synchronizedNavigableMap(new TreeMap<K, V>());
   *
   * // Needn't be in synchronized block
   * NavigableSet<K> set = map.navigableKeySet();
   *
   * synchronized (map) { // Synchronizing on map, not set!