ImmutableSortedMultiset.Builder<String> builder = ImmutableSortedMultiset.naturalOrder();
    assertThrows(IllegalArgumentException.class, () -> builder.setCount("a", -2));
  }

  public void testToImmutableSortedMultiset() {
    BiPredicate<ImmutableSortedMultiset<String>, ImmutableSortedMultiset<String>> equivalence =
        (ms1, ms2) ->
            ms1.equals(ms2)