Geek Thoughts: nomic chess
The rules of Nomic Chess start out like regular chess, except that when it is your turn, instead of making a move, you can change the rules such that any particular class of pieces (say pawns) can move like any other class of pieces (say queens).
An optional second rule is that when you are in check, you must make a move, not a rule change, in order to get out of check. (Otherwise it can be extremely difficult to arrive at a checkmate if any attacking piece can be turned into one that moves like a pawn or whatever).
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