code: def linked(base_dir: str, rr_id: str) -> str:
          
  
          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
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.