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