public function source($start = null, $end = null)
    {
        $source = '';
        $start = (int) $start;
        $end = $end === null ? ($this->count() - 1) : (int) $end;
        for ($i = $start; $i <= $end; $i++) {
            $source .= $this->_data[$i][1];
        }
        return $source;
    }