public void testCeilingEntry() {
    assertThat(create().ceilingEntry("a")).isNull();
  }

  public void testCeilingKey() {
    assertThat(create().ceilingKey("a")).isNull();
  }

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