public function addWorksheet($title)
    {
        $worksheet = $this->workbook->appendChild($this->createElement('Worksheet'));
        $title = preg_replace("/[\\\|:|\/|\?|\*|\[|\]]/", "", $title);
        $title = substr($title, 0, 31);
        $worksheet->setAttribute('ss:Name', $title);
        $worksheet->table = $worksheet->appendChild($this->createElement('Table'));
        $this->worksheets[md5($title)] = $worksheet;
        return $worksheet;
    }