private function write($str)
    {
        $this->source .= $this->indentation;
        if (func_num_args() == 1) {
            $this->source .= $str . "\n";
            return $this;
        }
        $this->source .= vsprintf($str, array_slice(func_get_args(), 1)) . "\n";
        return $this;
    }