code: def move_item_down(self, item):
          
          next_iter = self._next_iter_for(item)
          if next_iter is not None:
              self.model.swap(self._iter_for(item), next_iter)
docs: Move an item down in the list.
  
          Essentially swap it with the item below it.
  
          :param item: The item to be moved.