protected function buildSerial(){

        if(DEBUG_MODE){
            //in debug mode, use more meaningful serials
            $clazz = strtolower(get_class($this));
            $prefix = substr($clazz, strpos($clazz, 'taoqtiitem\\model\\qti\\') + 21).'_';
            $serial = str_replace('.', '', uniqid($prefix, true));
            $serial = str_replace('\\', '_', $serial);
        }else{
            //build a short unique id for memory saving
            $serial = uniqid('i');
        }

        return (string) $serial;
    }