code: def add(self, string: (str, list)):
          
          slot = _SlotFrame(self,
                            remove_callback=self._redraw,
                            entries=self._slot_columns)
          slot.add(string)
  
          self._slots.append(slot)
  
          self._redraw()
docs: Add a new slot to the multi-frame containing the string.
          :param string: a string to insert
          :return: None