@Test
public void testFlatten() {

    final Set<Set<String>> set = new HashSet<>();
    set.add(new HashSet<>(Arrays.asList("a", "b")));
    set.add(new HashSet<>(Arrays.asList("b", "c", "d")));
    final Collection<String> flattened = ACollectionHelper.flatten(set);
    assertEquals(5, flattened.size ());
    final List<String> flattenedList = new ArrayList<>(flattened);
    Collections.sort (flattenedList);
    assertEquals(Arrays.asList("a", "b", "b", "c", "d"), flattenedList);
}