@Test
public void testSingletons() {

    List<Poset<String>> strings = Poset.singletons(Arrays.asList("a", "b", "c"));
    assertEquals(strings.size(), 3);
    assertEquals(strings.get(0), A);
    
    exception.expect(java.lang.IllegalArgumentException.class);
    strings = Poset.singletons(Arrays.asList("a", "a"));
}