I have an outpost on the front lines, so walls occasionally get destroyed. The blueprint scanner detects the ghosts of destroyed buildings, just as a bot is replacing them, and triggers a build request. At the moment, there's no way to prevent a slight overflow of building materials from getting loaded to the train and delivered to the outpost, so the storage at the outpost slowly fills up with walls until it's overflowing.
I could deal with this by building a separate "trash" station at the outpost and siphoning off walls, but to me, it seems more elegant to just not trigger the build request in the first place, and just use the extra walls that have already been delivered anyway.
I've tried to do this by taking the logistics inventory off a roboport, inverting it (each * -1 -> each), and feeding it to the green channel on the Outpost Train Stop, but unfortunately any signal triggers a build request.
I've also tried adding that negative value to the ghost scanner's output, then filtering that sum with a decider combinator (each > 0 -> each input count). This correctly prevents requests from being generated, but unfortunately the ghost scanner isn't connected directly to the outpost, so other functionality (stuff intended to "refresh" the ghost scanner) fails.
I suspect that there's a way to hook the ghost scanner up directly and also have a circuit network contraption subtract any matching items in the logistic network, but there would be at least a few ticks of delay, so presumably a build request would still be generated, invalidating the in-game solution.
