def additional_special_tokens(self):
        """ All the additional special tokens you may want to use (list of strings). Log an error if used while not having been set. """
        if self._additional_special_tokens is None:
            logger.error("Using additional_special_tokens, but it is not set yet.")
        return self._additional_special_tokens
 All the additional special tokens you may want to use (list of strings). Log an error if used while not having been set. 
xtune/src/transformers/tokenization_utils.py