code: def set_extra_selections(self, key, extra_selections):
          
          # use draw orders to highlight current_cell and current_line first
          draw_order = DRAW_ORDERS.get(key)
          if draw_order is None:
              draw_order = DRAW_ORDERS.get('on_top')
  
          for selection in extra_selections:
              selection.draw_order = draw_order
  
          self.clear_extra_selections(key)
          self.extra_selections_dict[key] = extra_selections
docs: Set extra selections for a key.
  
          Also assign draw orders to leave current_cell and current_line
          in the backgrund (and avoid them to cover other decorations)
  
          NOTE: This will remove previous decorations added to  the same key.
  
          Args:
              key (str) name of the extra selections group.
              extra_selections (list of sourcecode.api.TextDecoration).