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Pekka Karjalainen invented this variant. A random square is marked after each half-move. If this is the first time it has been marked, it turns darker gray. If this is the second time, it turns black. You must move a unit off a black square at the first opportunity or it will disappear. Units cannot move to black squares. Line movers, pawns on the second rank, and castling king may still move across black squares. Line moves give check across black squares.
Thanks to Andrew Maxson for help with the debugging (January 2007).
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!