class HelloWorldRoute(...) 
  : Route

class HelloWorldViewModel :
  ViewModel<HelloWorldRoute>() { ... }

@Composable
fun HelloWorldScreen(
    viewModel: HelloWorldViewModel
) { ... }

val helloWorldRouter = router(
   HelloWorldViewModel::class
) {
    withView(::HelloWorldScreen)
    ... 
}


MyActivity.routerController = RouterController(
   helloWorldRouter,
   ...
)

MyActivity.onCreate() {
   ...
   routerController.open(HelloWorldRoute(...))
   setContent { 
      MaterialTheme { 
          RouterControllerContainer(
             routerController
          ) 
      } 
   }
}

class AppState(
    val screens: Stack<Screen>
)

inline fun <reified T : Screen> State.updateScreen(
    id: ScreenId,
    how: (T) -> T
): State = ...

fun State.swapScreens(
    i: Int,
    j: Int = screens.lastIndex
)State {
...
}

fun State.pushScreen(
    screen: Screen
)State = ...

fun State.popScreen(): State = ...

private fun Activity.render(screen: Screen, onMessage: (Message) -> Unit) =
    setContent {
        App(screen, onMessage) {
            Screen(screen, onMessage)
        }
    }