With that the var wont unset, and plus it decreases by 10 each time. They wanted to decrease by 10 the first time, then decrease every second.

//set %var 50 | dec %var 10 | dec -z %var

Set the variable %var with a value of 50, takes 10 from it with "dec %var 10" then decreases every second with the -z switch.

-Andy