code: def _assert_link_secret(self, action: str):
          
  
          if self._link_secret is None:
              LOGGER.debug('HolderProver._assert_link_secret: action %s requires link secret but it is not set', action)
              raise AbsentLinkSecret('Action {} requires link secret but it is not set'.format(action))
docs: Raise AbsentLinkSecret if link secret is not set.
  
          :param action: action requiring link secret