fun map (f, l) =
    match (l) { N -> N | C (x, xs) -> C (f (x), map (f, xs)) }

fun concat (l1, l2) =
    match (l1) { N -> l2 | C (x, xs) -> C (x, concat (xs, l2)) }

fun flatten (l) =
    match (l) { N -> N | C (l, ls) -> concat (l, flatten (ls)) }

fun return (x) = C (x, N)

fun bind (x, f) = flatten (map (f, x))

let fail = N

fun some d = C (d, N)

fun range (start, stop) =
    if (start >? stop) then { N } else { C (start, range (start + 1, stop)) }

fun all_sums (start, stop, n) =
    bind (range (start, stop), \x ->
    bind (range (start, stop), \y ->
    if (x + y =? n) then { return (some (x, y, n)) } else { fail }
    ))

fun show_sum (x, y, n) =
    print_int (x);
    print_string (" + ");
    print_int (y);
    print_string (" = ");
    print_int (n);
    print_string ("\n")

fun iter (f, l) =
    match (l) { N -> nothing | C (x, xs) -> f (x); iter (f, xs) }

let test =
    iter (show_sum, flatten (all_sums (13, 73, 73)))