'Return rId key of relationship of *reltype* to *target*, from an
existing relationship if there is one, otherwise a newly created one.'

def relate_to(self, target, reltype, is_external=False):

    if is_external:
        return self.rels.get_or_add_ext_rel(reltype, target)
    else:
        rel = self.rels.get_or_add(reltype, target)
        return rel.rId
