code: 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
docs: Return a newly added |_Relationship| instance.