class ToDoListHub(val lists: Map<User, List<ToDoList>>): ZettaiHub {
    override fun getList(user: User, listName: ListName): ToDoList? =
        lists[user]
        ?.firstOrNull { it.listName == listName }
}

data class Zettai(val hub: ZettaiHub): HttpHandler{
    // 메서드의 다른 부분
    fun fetchListContent(listId: Pair<User, ListName>): ToDoList =
        hub.getList(listId.first, listId.second)
        ?: error("List unknown")
}