docs: def get_subs_by_uid(self, uid):
          """
          Find all subject identifiers that is connected to a User ID.
  
          :param uid: A User ID
          :return: A set of subject identifiers
          """
          res = set()
          for sid in self.get('uid2sid', uid):
              res |= set(self.get('sid2sub', sid))
          return res