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

core/vendor/stack/builder/README.md

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+# Stack/Builder
+
+Builder for stack middlewares based on HttpKernelInterface.
+
+Stack/Builder is a small library that helps you construct a nested
+HttpKernelInterface decorator tree. It models it as a stack of middlewares.
+
+## Example
+
+If you want to decorate a [silex](https://github.com/fabpot/Silex) app with
+session and cache middlewares, you'll have to do something like this:
+
+    use Symfony\Component\HttpKernel\HttpCache\Store;
+
+    $app = new Silex\Application();
+
+    $app->get('/', function () {
+        return 'Hello World!';
+    });
+
+    $app = new Stack\Session(
+        new Symfony\Component\HttpKernel\HttpCache\HttpCache(
+            $app,
+            new Store(__DIR__.'/cache')
+        )
+    );
+
+This can get quite annoying indeed. Stack/Builder simplifies that:
+
+    $stack = (new Stack\Builder())
+        ->push('Stack\Session')
+        ->push('Symfony\Component\HttpKernel\HttpCache\HttpCache', new Store(__DIR__.'/cache'));
+
+    $app = $stack->resolve($app);
+
+As you can see, by arranging the layers as a stack, they become a lot easier
+to work with.
+
+In the front controller, you need to serve the request:
+
+    use Symfony\Component\HttpFoundation\Request;
+
+    $request = Request::createFromGlobals();
+    $response = $app->handle($request)->send();
+    $app->terminate($request, $response);
+
+Stack/Builder also supports pushing a `callable` on to the stack, for situations
+where instantiating middlewares might be more complicated. The `callable` should
+accept a `HttpKernelInterface` as the first argument and should also return a
+`HttpKernelInterface`. The example above could be rewritten as:
+
+    $stack = (new Stack\Builder())
+        ->push('Stack\Session')
+        ->push(function ($app) {
+            $cache = new HttpCache($app, new Store(__DIR__.'/cache'));
+            return $cache;
+        });
+
+## Inspiration
+
+* [Rack::Builder](http://rack.rubyforge.org/doc/Rack/Builder.html)
+* [HttpKernel middlewares](https://igor.io/2013/02/02/http-kernel-middlewares.html)