Rack::Builder.new do
  use added_middleware1
  use added_middleware2
end.call(@env) if @request.path == "/something"

use CommonMiddleware
map '/something' do
  use SpecialMiddleware
  run TheApp.new
end
map '/' do
  run TheApp.new
end

def initialize app, special_middleware
  @app, @special_middleware = app, special_middleware
end

def call env
  if route_matches?(env)
    @special_middleware.new(@app).call(env) # assumes special_middleware initializer takes no extra parameters
  else
    @app.call(env)
  end
end

def route_matches? env
  # examine the rack environment hash, return true or false
end

use RouteMatchingMiddleware, SpecialMiddleware
run TheApp.new

app = Rack::Builder.new {
   use Rack::CommonLogger
   use Rack::ShowExceptions
   map "/lobster" do
     use Rack::Lint
     run Rack::Lobster.new
   end
 }