@Test
public void testContainsAll() {

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