subset.clear();
    assertThat(elementSet).containsExactly("a", "f").inOrder();
    assertThat(subset).isEmpty();
    assertEquals(3, ms.size());
  }

  public void testCustomComparator() throws Exception {
    Comparator<String> comparator =
        new Comparator<String>() {
          @Override
          public int compare(String o1, String o2) {
            return o2.compareTo(o1);