@staticmethod<EOL><INDENT>def linked(base_dir: str, rr_id: str) -> str:<DEDENT>
cd_id = rev_reg_id2cred_def_id(rr_id)<EOL>link = join(base_dir, cd_id, rr_id)<EOL>return join(base_dir, cd_id, readlink(link)) if islink(link) else None<EOL>
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.