code: func (snap *ZoneSnapshot) GetRecordsForSerial(zone string, serial int) []*endpoint.Endpoint {
  	lastSerial, ok := snap.serials[zone]
  	if !ok {
  		// no mapping
  		return nil
  	}
  
  	if lastSerial != serial {
  		// outdated mapping
  		return nil
  	}
  
  	endpoints, ok := snap.endpoints[zone]
  	if !ok {
  		// probably a bug
  		return nil
  	}
  
  	return endpoints
  }
docs: // GetRecordsForSerial retrieves from memory the last known recordset for the (zone, serial) tuple