private function validateExtensions(\DOMDocument $dom, $file)
    {
        foreach ($dom->documentElement->childNodes as $node) {
            if (!$node instanceof \DOMElement || 'http://symfony.com/schema/dic/services' === $node->namespaceURI) {
                continue;
            }

            // can it be handled by an extension?
            if (!$this->container->hasExtension($node->namespaceURI)) {
                $extensionNamespaces = array_filter(array_map(function ($ext) { return $ext->getNamespace(); }, $this->container->getExtensions()));
                throw new InvalidArgumentException(sprintf(
                    'There is no extension able to load the configuration for "%s" (in %s). Looked for namespace "%s", found %s',
                    $node->tagName,
                    $file,
                    $node->namespaceURI,
                    $extensionNamespaces ? sprintf('"%s"', implode('", "', $extensionNamespaces)) : 'none'
                ));
            }
        }
    }