bad_prepend.sml

fun prepend(xs: int list, ys: int list)=
  if null xs
  then ys
  else (hd xs):: prepend( (tl xs), ys);

val xs = [4,5,6];
val ys = [1,2,3];

(* [4,5,6, 1,2,3] *)
prepend(xs, ys);
val xs = [7,8,9];

(* [7,8,9, 1,2,3] *)
prepend(xs, ys);