docs: 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
          """
  
          LOGGER.debug('Tails.current_rev_reg_id >>> base_dir: %s, cd_id: %s', base_dir, cd_id)
  
          if not ok_cred_def_id(cd_id):
              LOGGER.debug('Tails.current_rev_reg_id <!< Bad cred def id %s', cd_id)
              raise BadIdentifier('Bad cred def id {}'.format(cd_id))
  
          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))
  
          rv = rev_reg_id(cd_id, str(max(tags)))  # ensure 10 > 9, not '9' > '10'
          LOGGER.debug('Tails.current_rev_reg_id <<< %s', rv)
          return rv