public function normalize(ConfigurationInterface $configuration)
    {

        // normalize the configuration node without children
        $node = parent::normalize($configuration);

        // now we add recursive normalization
        foreach ($configuration->getChildren() as $child) {
            $node->{$configuration->getNodeName()}->children[] = $this->normalize($child);
        }

        // return the normalized node instance
        return $node;
    }