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

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