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

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

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