fun gcd(x,y) = let val p_x= ref x; val p_y = ref y; val temp = ref 0 in
while not (!p_x = !p_y) do
(if (!p_x > !p_y) then (temp := !p_x; p_x := !p_y ; p_y := (!temp - !p_y)) else p_y := (!p_y - !p_x));
!p_x
end;