assertTrue(descendingKeySet instanceof SynchronizedNavigableSet);
    assertThat(((SynchronizedNavigableSet<String>) descendingKeySet).mutex).isSameInstanceAs(mutex);
  }

  public void testDescendingMap() {
    NavigableMap<String, Integer> map = create();
    NavigableMap<String, Integer> descendingMap = map.descendingMap();
    assertTrue(descendingMap instanceof SynchronizedNavigableMap);