SetMultimap<String, Integer> unmod = Multimaps.unmodifiableSetMultimap(mod);
    assertThat(unmod).isNotSameInstanceAs(mod);
    assertThat(Multimaps.unmodifiableSetMultimap(unmod)).isSameInstanceAs(unmod);
    ImmutableSetMultimap<String, Integer> immutable =
        ImmutableSetMultimap.of("a", 1, "b", 2, "a", 3);
    assertThat(Multimaps.unmodifiableSetMultimap(immutable)).isSameInstanceAs(immutable);