open System;;

type 'a stack = Emp | Apnd of 'a stack * 'a;; // 'Apnd' for "append"

let s = Apnd(Apnd(Apnd(Emp,2),3),5);; //instead of Cons(2,Cons(3,Cons(5,Emp)));

let rec tolist = function
  | Emp -> []
  | Apnd(s,a) -> a::tolist(s);;

let rec tolist2 ax = function
  | Emp -> ax    // ax is the accumulator
  | Apnd(s,a) -> tolist2 (a::ax) s;;

let sl = tolist2 [] s;;

Console.WriteLine(sl);;

let tolist3 (M:'a stack) =
   let mutable L:'a list = []  // type annotation is needed here
   let mutable N = M
   while N <> Emp do   //  <> is "not equals":  != is used for something else
      match N with
        | Apnd(s,x) ->
            L <- x::L
            N <- s
        |_ -> ()
   L;;  // I *think* this works