module NonEmptyList = struct
  type 'a t = Cons of 'a * 'a t | Nil of 'a

  let rec from_list l = match l with
  |[]-> None
  |x::xs ->
    begin match from_list xs with
    | None -> Some (Nil a)
    | Some r ->  Some(Cons (x , r))
    end

end