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)

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 (C ((x, y, n), N)) } else { N }
    ))

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)))