fun zip (x) {
  case x of Pair (x, y) ->
    case x of
      Nil          -> return Nil
    | Cons (x, xs) -> case y of
                         Nil -> return Nil
  		       | Cons (y, ys) -> return Cons (Pair (x, y), zip (Pair (xs, ys)))
		       esac
    esac
  esac
}

fun unzip (x) {
  case x of
    Nil                    -> return Pair (Nil, Nil)
  | Cons (Pair (x, y), tl) ->
       case unzip (tl) of
         Pair (xs, ys) -> return Pair (Cons (x, xs), Cons (y, ys))
       esac
  esac
}

fun printList (l) {
  case l of
    Nil -> skip
  | Cons (x, xs) -> write (x); printList (xs)
  esac
}

z := read ();

x := Cons (1, Cons (2, Cons (3, Nil)));
y := Cons (100, Cons (200, Cons (300, Nil)));

case unzip (zip (Pair (x, y))) of
  Pair (x, y) -> printList (x); printList (y)
esac