POST adds, it does not replace.
so you an do it. However the logical choice if to check the same condition on the formula where you POST and not do it.
I agree with you and that is what I was trying to do: if condition then post(x,y) else "do nothing"
OR: if not condition then "do nothing" else post(x,y)
Replacing "do nothing" by 0 does not work, so back to my original question: how to "do nothing"?