public function updateGoodsQty($invNo, $artNo, $qty)
    {
        $this->_checkInvNo($invNo);
        $this->_checkQty($qty);
        $this->_checkArtNo($artNo);

        $digestSecret = self::digest(
            $this->colon($invNo, $artNo, $qty, $this->_secret)
        );

        $paramList = array(
            $this->_eid,
            $digestSecret,
            $invNo,
            $artNo,
            $qty
        );

        self::printDebug('update_goods_qty', $paramList);

        return $this->xmlrpc_call('update_goods_qty', $paramList);
    }