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);