(* rev1 reverses A[i] through A[n-i-1] *)
fun rev1(A,n,i) =
        if 2*i+1 >= n then ()
        else
            let
                val temp = Array.sub(A,i);
            in
                (
                    Array.update(A,i,Array.sub(A,n-i-1));
                    Array.update(A,n-i-1,temp);
                    rev1(A,n,i+1)
                )
            end;

fun reverse(A,n) = rev1(A,n,0);