fun square x = x * x;
fun fact x = if x=0 then 1
             else x * fact (x-1);
fun sum []      = 0           
  | sum (x::xs) = x + sum xs; 

fun length []      = 0
  | length (x::xs) = 1 + length xs;

fun append []      ys = ys
  | append (x::xs) ys = x :: append xs ys; 

fun map f []      = []
  | map f (x::xs) = f x :: map f xs;