code: func (ss *SQLSubStore) getOrCreateAcksPending(subid, seqno uint64) *sqlSubAcksPending {
  	if !ss.cache.needsFlush {
  		ss.cache.needsFlush = true
  		ss.sqlStore.scheduleSubStoreFlush(ss)
  	}
  	ap := ss.cache.subs[subid]
  	if ap == nil {
  		ap = &sqlSubAcksPending{
  			msgToRow: make(map[uint64]*sqlSubsPendingRow),
  			ackToRow: make(map[uint64]*sqlSubsPendingRow),
  			msgs:     make(map[uint64]struct{}),
  			acks:     make(map[uint64]struct{}),
  		}
  		ss.cache.subs[subid] = ap
  	}
  	if seqno > ap.lastSent {
  		ap.lastSent = seqno
  	}
  	return ap
  }
docs: // This returns the structure responsible to keep track of
  // pending messages and acks for a given subscription ID.