multiset.add("bar", 3);
    multiset.add("foo", 2);
    assertThat(multiset).containsExactly("foo", "foo", "bar", "bar", "bar").inOrder();
  }

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