u probably don't understand what i want, collective does
what i want to do is to /inc %var , my question was why it doesn't work when i put $null before the %var ($null should evaluated first). the $null was just an example, it can be:
inc $iif(!%a,-u4) %a
here, when the $iif will return $null it won't work.
i can do % $+ a at the end but i don't understand why %a doesn't work.