code: def links(base_dir: str, issuer_did: str = None) -> set:
          
  
          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)))}
docs: 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