NavigableMap<String, String> delegate = populate(new TreeMap<>(), entries);
                return Collections.synchronizedNavigableMap(delegate);
              }
            })
        .named("synchronizedNavigableMap/TreeMap, natural")
        .withFeatures(
            MapFeature.GENERAL_PURPOSE,
            MapFeature.ALLOWS_NULL_VALUES,

    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, @CheckForNull 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, @CheckForNull Object mutex) {