private val backStackMap: MutableMap<Any, BackStack<*>> =
    mutableMapOf()

class BackStack<T>(initialElement: T)

@Composable
private fun <T> fetchBackStack(key: String, defaultElement: T, override: T?): BackStack<T> {
    // line A
    val existing = backStackMap[key] as BackStack<T>?
    return when {
        // line B
        override != null -> BackStack(override)
        existing != null -> existing
        else -> BackStack(defaultElement)
    }.also {
        backStackMap[key] = it
    }
}