@Override
  public boolean add(K k)
  {
    return _map.put(k, PRESENT) == null;
  }
@Test
  public void testAdd()
  {
    final CowSet<String> set = new CowSet<>();

    Assert.assertEquals(set.size(), 0);
    Assert.assertFalse(set.contains("test"));

    Assert.assertTrue(set.add("test"));
    Assert.assertTrue(set.contains("test"));
    Assert.assertEquals(set.size(), 1);

    Assert.assertFalse(set.add("test"));
    Assert.assertTrue(set.contains("test"));
    Assert.assertEquals(set.size(), 1);
  }