Params support in `use`

[pke]

I am not sure if that is already supported and I just don't see it. I want to "dynamically" mount a koa app with routes to a variable route like this:

const appRouter = () => {
  return new Router()
  .use(ctx => {
    // has ctx.params.app
    // has ctx.params.id if its specified in the route,
  })
  .get("/:id", ctx => {
    // has ctx.params.app
    // has ctx.params.id,
  })
  .middleware()

app.use(mount("/:app", appRouter()))

Which should enable routes like:

/myapp/1 /myapp/2

Even better if I could design the approuter to be totally unaware of the /:app part of the route and only would have to deal with its own routes. Then I would like to move the processing of potential id params to a middleware that is called before the approuter and sets some infos in the ctx before passing control to the approuter.