@Test
public void testAdd() {

  set.add("hei");
  Assert.assertTrue("set with 1 element thinks it's empty", !set.isEmpty());
  Assert.assertTrue("set size != 1", set.size() == 1);
  Assert.assertTrue("add thinks object just added is not contained",
             !set.add("hei"));
  Assert.assertTrue("set with 1 element thinks it's empty", !set.isEmpty());
  Assert.assertTrue("set size != 1", set.size() == 1);
  Assert.assertTrue("set thinks new object is already contained",
  set.add("hei2"));
  Assert.assertTrue("set size != 2", set.size() == 2);
  Assert.assertTrue("add thinks object just added is not contained",
             !set.add("hei"));
  Assert.assertTrue("add thinks object just added is not contained",
             !set.add("hei2"));
}