assertSame(MUTEX, ((SynchronizedNavigableSet<String>) descendingSet).mutex);
  }

  public void testHeadSet_E() {
    NavigableSet<String> map = create();
    SortedSet<String> headSet = map.headSet("a");
    assertTrue(headSet instanceof SynchronizedSortedSet);
    assertSame(MUTEX, ((SynchronizedSortedSet<String>) headSet).mutex);
  }

  public void testHeadSet_E_B() {

    NavigableSet<E> outer = Synchronized.navigableSet(inner, null);
    inner.mutex = outer;
    return outer;
  }

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

    TestSet(NavigableSet<E> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

    @Override
    protected NavigableSet<E> delegate() {