To explain, the node taking input from IN.S passes it to the node below, then determines which of the nodes on either side horizontally need to accept input and pass it to the node below. Both IN.A and IN.B decide from the first node's instructions whether to pass their input or dispose of it. Then, the central node on the second line accepts the proper input, processes it in the case that IN.S sends 0, and sends it to the node below, which passes it to the output.

It's "slow" but it took forever to get this working how I wanted it to.