Support lexicon joins. Lexicon joins are a special case of foreign key joins where the key is one of the S, P, O or C columns of a statement index.
Right now all joins are self joins. We need foreign key joins to support joins against the RDF Lexicon (to materialize RDF binding set values) and for truth maintenance (to chase proof chains and resolve their antecedents to told triples in the KB). This will also take us some distance towards full datalog support.