ImmutableListMultimap.<String, Character>builder()
            .put("foo", 'f')
            .put("foo", 'o')
            .put("foo", 'o')
            .put("poo", 'p')
            .put("poo", 'o')
            .put("poo", 'o')
            .build();
    assertSame(multimap.inverse(), multimap.inverse());
    assertSame(multimap, multimap.inverse().inverse());
  }