so rules engines model if then relationships and in the "then" part its easy to insert new facts or take abitrary actions on those facts. What i'm not clear about, is how this model relates/overlaps with building a hierarchy of data, or if its orthogonal. Like, a common rules example is that my fathers brother is my uncle. But what about answering questions like, what are the names of all my friends, something that would be easy with datomic pull.