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

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