# File lib/rack/utils.rb, line 130
      def initialize app_f, app_r
        raise 'running context not provided' unless app_f
        raise 'running context does not respond to #context' unless app_f.respond_to? :context
        raise 'application context not provided' unless app_r
        raise 'application context does not respond to #call' unless app_r.respond_to? :call
        @for = app_f
        @app = app_r
      end