@Test
void testOfTwo() {

    final Set<List<String>> actual = Combination.of("a", "b")
        .collect(toSet());
    final Set<List<String>> expected = new HashSet<>(asList(
        singletonList("a"),
        singletonList("b"),
        asList("a", "b")
    ));
    assertEquals(expected, actual);
}