I would have done that:
declare
fun{Mirror X Y}
if X==0 then Y
else {Mirror (X div 10) (X mod 10)+Y*10}
end
end
{Browse {Mirror 1234 0}}
or, if you want only one argument:
declare
fun{Mirror X}
fun{Aux X Y}
if X==0 then Y
else {Aux (X div 10) (X mod 10)+Y*10}
end
end
in
{Aux X 0}
end
{Browse {Mirror 1234}}