@Test
public void testRetainAll() {

  Collection<String> list = Arrays.asList(new String[]{"apa", "apa2", "apa"});
  FoldedTreeSet<String> instance = new FoldedTreeSet<>();
  instance.addAll(list);
  assertEquals(false, instance.retainAll(list));
  assertEquals(2, instance.size());
  Collection<String> list2 = Arrays.asList(new String[]{"apa"});
  assertEquals(true, instance.retainAll(list2));
  assertEquals(1, instance.size());
}