public function display($level = 0)
    {
        $value = $this->getContent();

        if (null === $value) {
            $value = 'null';
        } elseif (is_object($value)) {
            $value = get_class($value);
        } elseif (is_array($value)) {
            $value = 'Array';
        }

        $ret = str_repeat(' ', $level * 4) . $value . "\n";

        $children = $this->getChildren();

        foreach ($children as $child) {
            $ret .= $child->display($level + 1);
        }

        return $ret;
    }