Return set of all paths to symbolic links (rev reg ids) associating
        their respective tails files, in specified base tails directory, on
        input issuer DID if specified.

        :param base_dir: base directory for tails files, thereafter split by cred def id
        :param issuer_did: issuer DID of interest
        :return: set of paths to symbolic links associating tails files

def links(base_dir: str, issuer_did: str = None) -> set:
        """
        Return set of all paths to symbolic links (rev reg ids) associating
        their respective tails files, in specified base tails directory, on
        input issuer DID if specified.

        :param base_dir: base directory for tails files, thereafter split by cred def id
        :param issuer_did: issuer DID of interest
        :return: set of paths to symbolic links associating tails files
        """

        return {join(dp, f) for dp, dn, fn in walk(base_dir) for f in fn
            if islink(join(dp, f)) and (not issuer_did or f.startswith('{}:4:'.format(issuer_did)))}