docs: def _zadd(self, key, pk, ts=None, ttl=None):
          """Redis lua func to add an event to the corresponding sorted set.
  
          :param key: the key to be stored in redis server
          :param pk: the primary key of event
          :param ts: timestamp of the event, default to redis_server's
           current timestamp
          :param ttl: the expiration time of event since the last update
          """
          return self.r.eval(self.LUA_ZADD, 1, key, ts or self._time(), pk)