docs: def xml(self):
          """
          Serialize this relationship collection into XML suitable for storage
          as a .rels file in an OPC package.
          """
          rels_elm = CT_Relationships.new()
          for rel in self.values():
              rels_elm.add_rel(
                  rel.rId, rel.reltype, rel.target_ref, rel.is_external
              )
          return rels_elm.xml