assertTrue(Multisets.retainOccurrences(multiset, toRetain));
    assertTrue(multiset.isEmpty());
  }

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