protected function extractNumberFormats($xmlReader)
    {
        while ($xmlReader->read()) {
            if ($xmlReader->isPositionedOnStartingNode(self::XML_NODE_NUM_FMT)) {
                $numFmtId = intval($xmlReader->getAttribute(self::XML_ATTRIBUTE_NUM_FMT_ID));
                $formatCode = $xmlReader->getAttribute(self::XML_ATTRIBUTE_FORMAT_CODE);
                $this->customNumberFormats[$numFmtId] = $formatCode;
            } else if ($xmlReader->isPositionedOnEndingNode(self::XML_NODE_NUM_FMTS)) {
                // Once done reading "numFmts" node's children
                break;
            }
        }
    }