@CollectionSize.Require(ZERO)
  public void testEmptyMapLast() {
    assertNull(navigableMap.lastEntry());
  }

  @MapFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(ZERO)
  public void testEmptyMapPollLast() {
    assertNull(navigableMap.pollLastEntry());
  }

  @CollectionSize.Require(ONE)
  public void testSingletonMapFirst() {
    assertEquals(a, navigableMap.firstEntry());
  }