@Override
   public boolean containsAll(IntSet set) {
      return set.isEmpty();
   }
@Test
   public void testContainsAll1() throws Exception {
      Set<Integer> hashSet = new HashSet<>();

      hashSet.add(3);

      assertFalse(es.containsAll(hashSet));
      assertTrue(hashSet.containsAll(es));

      hashSet.remove(3);

      assertTrue(hashSet.containsAll(es));
      assertTrue(es.containsAll(hashSet));
   }