symfony-notifications/NotificationsBundle/DependencyInjection/Configuration.php

<?php

namespace Bluesquare\NotificationsBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

class Configuration implements ConfigurationInterface
{
    /**
     * Generates the configuration tree builder.
     *
     * @return \Symfony\Component\Config\Definition\Builder\TreeBuilder The tree builder
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $root = $treeBuilder->root('billing');
        $root->end();
        return $treeBuilder;
    }
}