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