then:
        hierarchy.size() == 5
        hierarchy.iterator().next() == leaf
        assertBothExistsAndOneIsBeforeOther(hierarchy, middle1, root1);
        assertBothExistsAndOneIsBeforeOther(hierarchy, middle2, root2);
    }

    private static void assertBothExistsAndOneIsBeforeOther(Set<Configuration> hierarchy, Configuration beforeConf, Configuration afterConf) {
        assert hierarchy.contains(beforeConf)