KSRM, I believe the way the operation would usually be done on a Mill, first you have an unconditional increment of a, then a conditional select, the select being a “zero time” operation.
(a+1 a b)
(a a+1 a b) or (a+1 a+1 a b)
A given operation always adds the same count of results to the front of the belt. If you use branching you need to make sure the belt is always set up correctly before your branches merge.
Reply To: The Belt PeterH