code: func (ss *FileSubStore) AckSeqPending(subid, seqno uint64) error {
  	ss.Lock()
  	ss.updateSub.ID, ss.updateSub.Seqno = subid, seqno
  	if err := ss.writeRecord(nil, subRecAck, &ss.updateSub); err != nil {
  		ss.Unlock()
  		return err
  	}
  	si := ss.subs[subid]
  	if si != nil {
  		s := si.(*subscription)
  		delete(s.seqnos, seqno)
  		// Test if we should compact
  		if ss.shouldCompact() {
  			ss.fm.closeFileIfOpened(ss.file)
  			ss.compact(ss.file.name)
  		}
  	}
  	ss.Unlock()
  	return nil
  }
docs: // AckSeqPending records that the given message seqno has been acknowledged
  // by the given subscription.