code: def add(self, text, checked=False, sort=None):
          
          node = ListItem(parent_id=self.id, parent_server_id=self.server_id)
          node.checked = checked
          node.text = text
          if sort is not None:
              node.sort = sort
          self.append(node, True)
          self.touch(True)
          return node
docs: Add a new item to the list.
  
          Args:
              text (str): The text.
              checked (bool): Whether this item is checked.
              sort (int): Item id for sorting.