-"Capture" Chess (idea of opperwezen, maybe not new): captures are forced,
-but the goal is still to checkmate (not lose all material).
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-Cannibal Chess with forced captures.
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-Knightrelay: implement "official" version as Knightrelay v1
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-Antiking: implement v1 (deterministic) https://www.chessvariants.com/diffobjective.dir/anti-king-chess.html