code: def get_message_event_definition(self, messageEventDefinition):
          
          messageRef = first(self.xpath('.//bpmn:messageRef'))
          message = messageRef.get(
              'name') if messageRef is not None else self.node.get('name')
          return MessageEventDefinition(message)
docs: Parse the messageEventDefinition node and return an instance of
          MessageEventDefinition