exception ListEmpty;

datatype 'a cs52list = Nil
                     | Cons of 'a * 'a cs52list;

fun first Nil          = raise ListEmpty
  | first (Cons(x, _)) = x;

fun rest Nil           = raise ListEmpty
  | rest (Cons(_, xs)) = xs;

fun countUp k = Cons(k, countUp (k+1));