fun even(n) = if n = 0 then true else
                if n = 1 then false else even(n-2);

 fun append(x,y) = if null(x) then y else
                hd(x) :: append (tl(x),y);

     fun f(x) = if null(x) then [] else
                if even(hd(x)) then hd(x) ::  f(tl(x))
                else  hd(x)  ::f(tl(x));