code: def add_rel(self, rId, reltype, target, is_external=False):
          
          target_mode = RTM.EXTERNAL if is_external else RTM.INTERNAL
          relationship = CT_Relationship.new(rId, reltype, target, target_mode)
          self.append(relationship)
docs: Add a child ``<Relationship>`` element with attributes set according
          to parameter values.