assertEquals(0, diff.count("b"));
    assertEquals(1, diff.count("a"));
    assertFalse(diff.contains("b"));
    assertTrue(diff.contains("a"));
  }

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