@Test
public void testLowerKey() {

    IncrementalSortMap<Integer, String> q = map5();
    Object e1 = q.lowerKey(three);
    assertEquals(two, e1);
    Object e2 = q.lowerKey(six);
    assertEquals(five, e2);
    Object e3 = q.lowerKey(one);
    assertNull(e3);
    Object e4 = q.lowerKey(zero);
    assertNull(e4);
}