Generate Permutation in Ocaml

let insert_all n lst =
 let rec insert_all_rec result part = function 
  | []  -> (List.rev (n::part))::result 
  | (h::t) as lst -> 
     let new_lst =  (List.rev (n::part)) @ lst in
     insert_all_rec (new_lst :: result) (h::part) t 
 in 
   insert_all_rec [] [] lst ;;

val insert_all : 'a -> 'a list -> 'a list list = <fun>

let rec generate_permutation n =
 match n with
 | 0 -> []
 | 1 -> [[1]]
 | n -> let list_perm = generate_permutation (n-1) in
      let list_list_perm = List.map (insert_all n) list_perm in
       List.flatten list_list_perm ;;

val generate_permutation : int -> int list list = <fun>


generate_permutation 1 ;;
generate_permutation 2 ;;
generate_permutation 3 ;;
generate_permutation 4 ;;