assertTrue(descendingMap instanceof SynchronizedNavigableMap);
    assertSame(mutex, ((SynchronizedNavigableMap<String, Integer>) descendingMap).mutex);
  }

  public void testFirstEntry() {
    create().firstEntry();
  }

  public void testFirstKey() {
    NavigableMap<String, Integer> map = create();
    map.put("a", 1);
    map.firstKey();
  }