let rec ins z = function
 | [] -> [[z]]
 | x::xs ->
     let res = ins z xs
     let res1 = List.map (fun t -> x::t) res
     (z::x::xs)::res1

let rec ins z = function
| [] -> [[z]]
| x::xs as L ->
    (z::L)::(List.map (fun t -> x::t) (ins z xs))

ins 0 [1;2;3]

let rec perm = function
| [] -> [[]]
| x::xs ->
    let res = perm xs
    res |> List.map (fun t -> ins x t) |> List.concat

let rec perm = function
| [] -> [[]]
| x::xs ->
    perm xs |> List.collect (fun t -> ins x t)

perm [1;2;3]

let fact n = List.length(perm [1..n])

fact 5