public function creditPart($invNo, $credNo = "")
    {
        $this->_checkInvNo($invNo);
        $this->_checkCredNo($credNo);

        if ($this->goodsList === null || empty($this->goodsList)) {
            $this->_checkArtNos($this->artNos);
        }

        //function activate_part_digest
        $string = $this->_eid . ":" . $invNo . ":";

        if ($this->artNos !== null && !empty($this->artNos)) {
            foreach ($this->artNos as $artNo) {
                $string .= $artNo["artno"] . ":". $artNo["qty"] . ":";
            }
        }

        $digestSecret = self::digest($string . $this->_secret);
        //end activate_part_digest

        $paramList = array(
            $this->_eid,
            $invNo,
            $this->artNos,
            $credNo,
            $digestSecret
        );

        if ($this->goodsList !== null && !empty($this->goodsList)) {
            $paramList[] = 0;
            $paramList[] = $this->goodsList;
        }

        $this->artNos = array();

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

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