datatype naturali = zero | succ  of  naturali;

val n = succ (succ zero) ;

succ n;

val rec naturali_to_int = fn zero => 0
                        | succ (n) => 1 + naturali_to_int(n);

val rec int_to_naturali = fn 0 => zero
                        | n => succ ( int_to_naturali (n-1));

fun summ n m = case n of
            zero => m
            | succ (n) => summ (n) (succ m);

local
 fun per n m p = case n of
            zero =>  p
            | succ (n) => ( per (n) (m) (summ p  m) )
in
    fun mul n m = per n m zero
end;




naturali_to_int ( mul ( int_to_naturali (6) ) (int_to_naturali (9) )  );