public void testLowerEntry() {
    assertThat(create().lowerEntry("a")).isNull();
  }

  public void testLowerKey() {
    assertThat(create().lowerKey("a")).isNull();
  }

  public void testNavigableKeySet() {
    NavigableMap<String, Integer> map = create();
    NavigableSet<String> navigableKeySet = map.navigableKeySet();
    assertTrue(navigableKeySet instanceof SynchronizedNavigableSet);