diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index 6093271..58f6fb3 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -24,13 +24,13 @@
  * It consists of several steps, of which each are explained in more details
  * below:
  *   1. Get a collection of routes which potentially match the current request:
- *     This is done by the route provider ::getInitialRouteCollection()
+ *     This is done by the route provider, see ::getInitialRouteCollection().
  *   2. Filter the collection down further more. For example this filters out
- *     routes applying to other formats: ::applyRouteFilters()
+ *     routes applying to other formats: See ::applyRouteFilters()
  *   3. Find the best matching route out of the remaining ones, by applying a
- *     regex: ::matchCollection()
+ *     regex: See ::matchCollection().
  *   4. Enhance the list of route attributes, for example loading entity objects
- *     : ::applyRouteEnhancers().
+ *     See ::applyRouteEnhancers().
  *
  * This implementation uses ideas of the following routers:
  *
@@ -48,26 +48,28 @@ class Router extends UrlMatcher implements RequestMatcherInterface, RouterInterf
   protected $routeProvider;
 
   /**
+   * The list of available enhancers.
+   *
    * @var \Symfony\Cmf\Component\Routing\Enhancer\RouteEnhancerInterface[]
    */
   protected $enhancers = [];
 
   /**
-   * Cached sorted list of enhancers
+   * Cached sorted list of enhancers.
    *
    * @var \Symfony\Cmf\Component\Routing\Enhancer\RouteEnhancerInterface[]
    */
   protected $sortedEnhancers;
 
   /**
-   * An array of RouteFilterInterface objects.
+   * The list of available route filters.
    *
    * @var \Symfony\Cmf\Component\Routing\NestedMatcher\RouteFilterInterface[]
    */
   protected $filters = [];
 
   /**
-   * Array of RouteFilterInterface objects, sorted.
+   * Cached sorted list route filters.
    *
    * @var \Symfony\Cmf\Component\Routing\NestedMatcher\RouteFilterInterface[]
    */
@@ -107,7 +109,7 @@ public function __construct(BaseRouteProviderInterface $route_provider, CurrentP
    *
    * @return $this
    */
-  public function addRouteEnhancer(BaseRouteEnhancerInterface $route_enhancer, $priority) {
+  public function addRouteEnhancer(BaseRouteEnhancerInterface $route_enhancer, $priority = 0) {
     $this->enhancers[$priority][] = $route_enhancer;
     return $this;
   }