assertFalse(Multisets.containsOccurrences(superMultiset, diffMultiset));
    assertTrue(Multisets.containsOccurrences(diffMultiset, subMultiset));
  }

  public void testRetainEmptyOccurrences() {
    Multiset<String> multiset = HashMultiset.create(asList("a", "b", "a"));
    Multiset<String> toRetain = HashMultiset.create();
    assertTrue(Multisets.retainOccurrences(multiset, toRetain));