Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/vendor/symfony-cmf/routing/ChainRouterInterface.php

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+<?php
+
+/*
+ * This file is part of the Symfony CMF package.
+ *
+ * (c) 2011-2014 Symfony CMF
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Cmf\Component\Routing;
+
+use Symfony\Component\Routing\RouterInterface;
+use Symfony\Component\Routing\Matcher\RequestMatcherInterface;
+
+/**
+ * Interface for a router that proxies routing to other routers.
+ *
+ * @author Daniel Wehner <dawehner@googlemail.com>
+ */
+interface ChainRouterInterface extends RouterInterface, RequestMatcherInterface
+{
+    /**
+     * Add a Router to the index.
+     *
+     * @param RouterInterface $router   The router instance. Instead of RouterInterface, may also
+     *                                  be RequestMatcherInterface and UrlGeneratorInterface.
+     * @param integer         $priority The priority
+     */
+    public function add($router, $priority = 0);
+
+    /**
+     * Sorts the routers and flattens them.
+     *
+     * @return RouterInterface[] or RequestMatcherInterface and UrlGeneratorInterface.
+     */
+    public function all();
+}