docs: def archive_url(self, entity_id, channel=None):
          '''Generate a URL for the archive of an entity..
  
          @param entity_id The ID of the entity to look up as a string
                 or reference.
          @param channel Optional channel name.
          '''
          url = '{}/{}/archive'.format(self.url, _get_path(entity_id))
          return _add_channel(url, channel)