assertTrue(navigableKeySet instanceof SynchronizedNavigableSet);
    assertSame(mutex, ((SynchronizedNavigableSet<String>) navigableKeySet).mutex);
  }

  public void testPollFirstEntry() {
    create().pollFirstEntry();
  }

  public void testPollLastEntry() {
    create().pollLastEntry();
  }

  public void testSubMap_k_k() {
    NavigableMap<String, Integer> map = create();