code: def linked(base_dir: str, rr_id: str) -> str:
          
  
          LOGGER.debug('Tails.linked >>> base_dir: %s, rr_id: %s', base_dir, rr_id)
  
          if not ok_rev_reg_id(rr_id):
              LOGGER.debug('Tails.linked <!< Bad rev reg id %s', rr_id)
              raise BadIdentifier('Bad rev reg id {}'.format(rr_id))
  
          cd_id = rev_reg_id2cred_def_id(rr_id)
          link = join(base_dir, cd_id, rr_id)
  
          rv = join(base_dir, cd_id, readlink(link)) if islink(link) else None
          LOGGER.debug('Tails.linked <<< %s', rv)
          return rv
docs: 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.