NavigableSet<String> delegate = new TreeSet<>(MinimalCollection.of(elements));
                return Collections.synchronizedNavigableSet(delegate);
              }
            })
        .named("synchronizedNavigableSet/TreeSet, natural")
        .withFeatures(
            SetFeature.GENERAL_PURPOSE,
            CollectionFeature.SERIALIZABLE,

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible // NavigableSet
  @VisibleForTesting
  static final class SynchronizedNavigableSet<E extends @Nullable Object>
      extends SynchronizedSortedSet<E> implements NavigableSet<E> {
    SynchronizedNavigableSet(NavigableSet<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

    @Override

    private static final long serialVersionUID = 0;
  }

  @GwtIncompatible // NavigableSet
  @VisibleForTesting
  static final class SynchronizedNavigableSet<E extends @Nullable Object>
      extends SynchronizedSortedSet<E> implements NavigableSet<E> {
    SynchronizedNavigableSet(NavigableSet<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

    @Override