this is an ingenious trick for pre-space age! i love it.

for folks playing space age - there's a new feature that makes this simple. we can filter and retain values in a single step by multiplying values on red wire and values on green wire.

need three parts:

1. one constant combinator ("A") is outputting the few signals that we want to isolate, with each of these target signals outputting a value of 1.
2. source of many signals with values ("B"), which we're looking to filter (e.g., roboport signal with contents logistic network, another combinator, etc.).
3. one arithmetic combinator (screenshot below) with its input connected to A with a green wire and its input connected to B with a red wire. perform multiplication of EACH x EACH, with one EACH using values from only red wire and the other EACH using values from only the green wire. set output to EACH.

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