@Test
public void testKeySet() {

    IncrementalSortMap<Integer, String> map = map5();
    Set s = map.keySet();
    assertEquals(5, s.size());
    assertTrue(s.contains(one));
    assertTrue(s.contains(two));
    assertTrue(s.contains(three));
    assertTrue(s.contains(four));
    assertTrue(s.contains(five));
}