code: def to_xml(self):
          
          element = etree.Element(self._tag_name)
          struct_to_xml(element, [
              {"author": self.handle},
              {"target_guid": self.target_guid},
              {"target_type": DiasporaRetraction.entity_type_to_remote(self.entity_type)},
          ])
          return element
docs: Convert to XML message.