@SuppressWarnings("unchecked")
  protected <E> NavigableSet<E> create() {
    TestSet<E> inner =
        new TestSet<>(new TreeSet<E>((Comparator<E>) Ordering.natural().nullsFirst()), null);
    NavigableSet<E> outer = Synchronized.navigableSet(inner, null);
    inner.mutex = outer;
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {

  @SuppressWarnings("unchecked")
  protected <E> NavigableSet<E> create() {
    TestSet<E> inner =
        new TestSet<>(new TreeSet<E>((Comparator<E>) Ordering.natural().nullsFirst()), MUTEX);
    NavigableSet<E> outer = Synchronized.navigableSet(inner, MUTEX);
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {