function call with multiple output

[shish274@…]

function add
input Integer a;
input Integer b;
output Integer sum;
output Integer subtract;
output Integer multiply;
algorithm
sum:= a+b;
if (a>b) then
subtract:= a-b;
else
subtract:= b-a;
multiply:= a*b;
end if;
end add;

if i want to get just the multiplication part of the result then i did

(_{mult) := add (3,4);}

Also, (add,sub):= add (3,4) works fine
but when i want only subtraction part then it doesnt work
(,sub):= add (3.4);