stab one at automatically finding the rack root app and adding flash …

…to it

example/base_app.ru

@@ -17,6 +17,6 @@ class Base
   end
 end
 use Rack::Session::Cookie
-use Rack::Flash, :flash_app_class => Base
+use Rack::Flash#, :flash_app_class => Base
 use Rack::ShowExceptions
 run Base.new

lib/rack/flash.rb

@@ -1,3 +1,20 @@
+module Rack
+  class Builder
+    attr :ins
+    def use(middleware, *args, &block)
+      middleware.instance_variable_set "@rack_builder", self
+      @ins << lambda { |app| 
+        middleware.new(app, *args, &block) 
+      }
+    end
+
+    def run(app)
+      app.class.instance_variable_set "@rack_builder", self
+      @ins << app #lambda { |nothing| app }
+    end
+  end
+end
+
 module Rack
   class Flash
     # Raised when the session passed to FlashHash initialize is nil. This
@@ -105,6 +122,11 @@ def initialize(app, opts={})
         app_class.class_eval do
           def flash; env['rack-flash'] end
         end
+      else
+        rack_root_app = self.class.instance_variable_get("@rack_builder").ins.last
+        def rack_root_app.flash
+          env['rack-flash']
+        end
       end
       @app, @opts = app, opts
     end