code: def current_rev_reg_id(base_dir: str, cd_id: str) -> str:
          
  
          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)))
docs: 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