private function findPosition($index){
        if ($index < $this->total){
            $this->letter[] = $this->alphabet[$index];
        }
        else {
            $mod = $index % $this->total;
            $div = (int) $index / $this->total;
            $this->letter[] = $this->alphabet[$mod];
            $this->findPosition($div-1);
        }
    }