public function autoVar($default = null)
    {
        $backtrace = debug_backtrace();
        $name = '';
        if (isset($backtrace[1]['class'])) {
            $name = $backtrace[1]['class'] . '::';
        }
        $name .= $backtrace[1]['function'];
        $name .= '(' . $backtrace[1]['line'] . ')';
        $var = new DocumentVar($this, $name);
        if (isset($default)) {
            $var->setDefault($default);
        }
        return $var;
    }