Return rId of external relationship of *reltype* to *target_ref*,
        newly added if not already present in collection.

def get_or_add_ext_rel(self, reltype, target_ref):
        """
        Return rId of external relationship of *reltype* to *target_ref*,
        newly added if not already present in collection.
        """
        rel = self._get_matching(reltype, target_ref, is_external=True)
        if rel is None:
            rId = self._next_rId
            rel = self.add_relationship(
                reltype, target_ref, rId, is_external=True
            )
        return rel.rId