code: def add(self, message_dict):
          
          uuid = message_dict[TASK_UUID_FIELD]
          if uuid in self._tasks:
              task = self._tasks[uuid]
          else:
              task = Task()
          task = task.add(message_dict)
          if task.is_complete():
              parser = self.transform(["_tasks", uuid], discard)
              return [task], parser
          else:
              parser = self.transform(["_tasks", uuid], task)
              return [], parser
docs: Update the L{Parser} with a dictionary containing a serialized Eliot
          message.
  
          @param message_dict: Dictionary of serialized Eliot message.
  
          @return: Tuple of (list of completed L{Task} instances, updated
              L{Parser}).