docs: def purge_archives(self):
          """
          Delete older archived items.
  
          Use the class attribute NUM_KEEP_ARCHIVED to control
          how many items are kept.
          """
  
          klass = self.get_version_class()
          qs = klass.normal.filter(object_id=self.object_id,
                                   state=self.ARCHIVED).order_by('-last_save')[self.NUM_KEEP_ARCHIVED:]
  
          for obj in qs:
              obj._delete_reverses()
              klass.normal.filter(vid=obj.vid).delete()