Multiset<String> ms2 = HashMultiset.create(asList("a"));
    assertThat(difference(ms1, ms2)).containsExactly("a", "b");
  }

  public void testDifferenceWithRemovedElement() {
    Multiset<String> ms1 = HashMultiset.create(asList("a", "b", "a"));
    Multiset<String> ms2 = HashMultiset.create(asList("b"));
    assertThat(difference(ms1, ms2)).containsExactly("a", "a");
  }