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I recently ran across the entry for Berkelian C in the ECV, which consists of one rule: When a piece is unobserved (i.e. not defended) it disappears from the board. I'm not sure this would result in much of a game. But here is my development of this. Any piece that is unobserved (except the King) becomes 'cloaked'. Cloaked pieces cannot capture or be captured. They can only make a non-capturing move. Then, if the piece becomes defended again it becomes a normal capturing piece. A further development: cloaked pieces' non-capturing moves are made secretly by the player moving them by writing down the square they have moved to. I don't think you could enforce this last rule via PBM however.