'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):

    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
