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class ScopedBackPressHandler(
    // nullable, because root level doesn’t have one
    var backStack: BackStack<*>? = null 
) {
    // remainder the same
}

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class Entry<T>(val element: T) {
    val children: MutableMap<Any, BackStack<*>> = mutableMapOf()
}

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A [back stack of 1 element]
└── B [back stack of 1 element]
      └── C [back stack of 1 element]
            └── D [back stack of 1 element]

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A [back stack of 2 elements]
└── X [...]
      └── Y [...]
            └── Z [...]

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A [back stack of 1 element]
└── B [back stack of 2 elements]
      └── C [back stack of 4 elements]
            └── D [back stack of 1 element]

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class MainActivity : AppCompatActivity() {
    private val rootHandler = ScopedBackPressHandler()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                RootBackHandler(rootHandler) {
                    Root.Content(LoggedOut)
                }

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@Composable
fun RootBackHandler(rootHandler: ScopedBackPressHandler, children: @Composable() () -> Unit) {
    val downstream = +memo { ScopedBackPressHandler() }
    val handleBackPressHere: () -> Boolean = { downstream.handle() }
    rootHandler.handlers.add(handleBackPressHere)
    +onDispose { rootHandler.handlers.remove(handleBackPressHere) }

    backPressHandler.Provider(value = downstream) {
        children()
    }

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@Composable
fun Content() {
    BackHandler(Routing.AlbumList as Routing) { backStack ->
        when (val currentRouting = backStack.last()) {
            // all Content() on the right hand side are @Composable
            is Routing.AlbumList -> AlbumList.Content()
            is Routing.PhotosOfAlbum -> PhotosOfAlbum.Content(currentRouting.album)
            is Routing.FullScreenPhoto -> FullScreenPhoto.Content(currentRouting..photo)
        }

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val handleBackPressHere: () -> Boolean = { downstream.handle() || backStack.pop() }

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@Composable
fun <T> BackHandler(routing: T, children: @Composable() (BackStack<T>) -> Unit) {
    // grab the parent handler, wherever it's up the tree
    val upstream = +ambient(backPressHandler)
    // to keep track of children down the tree
    val downstream = +memo { ScopedBackPressHandler() }
    // let's have a default back stack on this level
    val backStack =  BackStack(routing)
    // our lambda for handling back press: asking all children,
    val handleBackPressHere: () -> Boolean = { downstream.handle() || backStack.pop() }