let sign = function
  n when (n > 0) -> 1
| n when (n < 0) -> -1
| n -> 0


let rec sum_n = function
  0 -> 0
| n -> n + sum_n (n-1)

let sum_n_2 n =
  let rec list_n = function 
      0 -> []
    | n -> n :: (list_n (n-1)) in
  List.fold_left (+) 0 (list_n n)