public static function convertToNumberIfFraction(&$operand)
    {
        if (preg_match('/^'.self::STRING_REGEXP_FRACTION.'$/i', $operand, $match)) {
            $sign = ($match[1] == '-') ? '-' : '+';
            $fractionFormula = '='.$sign.$match[2].$sign.$match[3];
            $operand = PHPExcel_Calculation::getInstance()->_calculateFormulaValue($fractionFormula);
            return true;
        }
        return false;
    }