protected function _diffQuoteData($oldQuote, $newQuote)
    {
        if (empty($oldQuote)) {
            return $newQuote;
        }
        return $this->_diffBilling($oldQuote['billing'], $newQuote['billing']) +
            $this->_diffCoupon($oldQuote['coupon'], $newQuote['coupon']) +
            $this->_diffShipping($oldQuote['shipping'], $newQuote['shipping']) +
            $this->_diffSkus($oldQuote['skus'], $newQuote['skus']) +
            $this->_diffAmounts($oldQuote['amounts'], $newQuote['amounts']);
    }