def current_rev_reg_id(base_dir: str, cd_id: str) -> str:
        """
        Return the current revocation registry identifier for
        input credential definition identifier, in input directory.

        Raise AbsentTails if no corresponding tails file, signifying no such revocation registry defined.

        :param base_dir: base directory for tails files, thereafter split by cred def id
        :param cd_id: credential definition identifier of interest
        :return: identifier for current revocation registry on input credential definition identifier
        """

        tags = [int(rev_reg_id2tag(basename(f))) for f in Tails.links(base_dir)
            if cd_id in basename(f)]
        if not tags:
            raise AbsentTails('No tails files present for cred def id {}'.format(cd_id))

        return rev_reg_id(cd_id, str(max(tags)))