public void testPollFirstUnsupported() {
    assertThrows(UnsupportedOperationException.class, () -> navigableMap.pollFirstEntry());
  }

  @CollectionSize.Require(SEVERAL)
  public void testLower() {
    resetWithHole();
    assertEquals(null, navigableMap.lowerEntry(a.getKey()));
    assertEquals(null, navigableMap.lowerKey(a.getKey()));
    assertEquals(a, navigableMap.lowerEntry(b.getKey()));