Return rId key of relationship to *part*, from the existing
        relationship if there is one, otherwise a newly created one.

def relate_to(self, part, reltype):
        """
        Return rId key of relationship to *part*, from the existing
        relationship if there is one, otherwise a newly created one.
        """
        rel = self.rels.get_or_add(reltype, part)
        return rel.rId