ImmutableList<String> empty = ImmutableList.of();
    ImmutableList<String> a = ImmutableList.of("a");
    ImmutableList<String> aa = ImmutableList.of("a", "a");
    ImmutableList<String> ab = ImmutableList.of("a", "b");
    ImmutableList<String> b = ImmutableList.of("b");

    Helpers.testComparator(lexy, empty, a, aa, ab, b);

    new EqualsTester()
        .addEqualityGroup(lexy, ordering.lexicographical())