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Billiard-ball computing: Fredkin gate implemented directly

Another implementation of the Fredkin gate, this time built directly from interactions. This was trickier to do than the switch-gate version. You would think six interactions would work out simpler than four switch gates (containing eight interactions), but switch gates separate their inputs and outputs neatly whereas with interactions you have to add structure around them. Especially when you want reversibility so you have to preserve four output paths from an interaction (or four input paths when you use them in reverse).

It's fairly obvious that this could be made more compact, but I'm leaving it until I build better circuit-design tools.