let nats = {
  let aux = fn n -> n << @(aux (n + 1));
  aux 0 
}

let fold = fn f #acc (a << as) -> {
  let acc' = !#f acc a;
  acc' << @(fold f acc' !@as)
}

let triangles = fold #(fn x y -> x + y) 0 nats

let head = fn (hd << tl) -> hd 

let print_triangles = print_int (head triangles) << @print_triangles