Get, from the specified directory, the path to the tails file associated with
        the input revocation registry identifier, or None for no such file.

        :param base_dir: base directory for tails files, thereafter split by cred def id
        :param rr_id: rev reg id
        :return: (stringified) path to tails file of interest, or None for no such file.

def linked(base_dir: str, rr_id: str) -> str:
        """
        Get, from the specified directory, the path to the tails file associated with
        the input revocation registry identifier, or None for no such file.

        :param base_dir: base directory for tails files, thereafter split by cred def id
        :param rr_id: rev reg id
        :return: (stringified) path to tails file of interest, or None for no such file.
        """

        cd_id = rev_reg_id2cred_def_id(rr_id)
        link = join(base_dir, cd_id, rr_id)
        return join(base_dir, cd_id, readlink(link)) if islink(link) else None