fun <T> generate(n: Int, a: MutableList<T>, callback: (MutableList<T>) -> Unit) {
    if (n == 1) {
        callback(a)
    }
    else {
        val nIsEven = n % 2 == 0
        for (i in 0..(n - 2)) {
            generate(n - 1, a, callback)
            a.swap(n - 1, if (nIsEven) i else 0)
        }
        generate(n - 1, a, callback)
    }
}