fun showUserOrders(username: String, password: String) {
    login(username, password)
        .flatMap { user -> fetchUserOrders(user.userId) }
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe { showOrders(it) }
}

fun login(username: String, password: String): Single<User>

fun fetchUserOrders(userId: Long): Single<List<Orders>>