'Return a newly added |_Relationship| instance.'

def add_relationship(self, reltype, target, rId, is_external=False):

    rel = _Relationship(rId, reltype, target, self._baseURI, is_external)
    self[rId] = rel
    if (not is_external):
        self._target_parts_by_rId[rId] = target
    return rel
