def sep_token(self):
        """ Separation token (string). E.g. separate context and query in an input sequence. Log an error if used while not having been set. """
        if self._sep_token is None:
            logger.error("Using sep_token, but it is not set yet.")
        return self._sep_token
 Separation token (string). E.g. separate context and query in an input sequence. Log an error if used while not having been set. 
xtune/src/transformers/tokenization_utils.py