code: func (ss *SQLSubStore) addSeq(subid, seqno uint64) bool {
  	ap := ss.getOrCreateAcksPending(subid, seqno)
  	ap.msgs[seqno] = struct{}{}
  	return len(ap.msgs) >= sqlMaxPendingAcks
  }
docs: // Adds the given sequence to the list of pending messages.
  // Returns true if the number of pending messages has
  // reached a certain threshold, indicating that the
  // store should be flushed.