Wei-Hwa Huang, 25 Apr 1995
Two years ago I invented along with my friend Robert Au a game inspired by retroanalysis, which we called "Retro-chess." Unfortunately, we never finished a game due to its complexity. Here are the rules:
Start with an empty chessboard.
Black places his king on any square. White then places his king on any square not adjacent to the Black king. Play then alternates.
Each move must be a "backwards" move in time in a valid chess game. The winner is the first player to restore all 16 of his pieces to the starting position.
If either player proves that the position is unreachable in a standard chess game, then the player who has made the last "unmove" loses. A corollary is that a player may try to bluff and create an unreachable position, hoping that the opponent will not notice and make an unmove.
If after your unmove your King is in check, it is customary to say "kcehc" (pronounced "kech") to warn your opponent that he must uncheck your king his next move.
The Fifty-Move Law: A player loses if after fifty of his moves he has not uncaptured any of his opponent's pieces, and not vacated any of his opponent's starting squares. In other words, he does not lose if within the last fifty moves he has made an uncapture or has moved a piece away from the 16 squares on the other side of the board.
Perpetual Kcehc: A player may not Kcehc more than 10 moves in a row.
This may work for a play by e-mail game. Are there any contingencies that I have not planned?
This looks like playing Proca-Retractor ! Except that in Proca games, the goal is not to restore your men to their starting position (what if your opponent will not free your home squares ?) but to reach a position where a forward mate in 1 is possible. That is, the goal is to retract your last move and mate in one. If you can't mate in one then it is your opponent who will retract and try to mate in one.
The problem with starting with two lone Kings is to get back your men without which you can't mate the opponent. Clearly the opponent will not uncapture them unless forced.
I remember playing a few such games with Laurent Joudon. A usual trick was to bring the wh K to a1, retract -1. Kb2xBa1 a2-a1=B+ -2. Kb1-b2 and if your opponent will not retract b3xa2 then drive the bl.P up to a7, start the same trick again and eventually you'll get some of your men back. (Of course all these maneuvers have been discovered by the 1st composers of Proca-retractors problems, perhaps Zeno Proca himself !!!)
Quite soon interesting legality problems arise, and we had to spend more and more time between our moves, simply sorting out the position. I remember it was very funny even when the games were aborted due to lack of time.
> This may work for a play by e-mail game. Are there any > contingencies that I have not planned?
Certainly! Imagine a World RetroChess Championship, between G. Vorapsak and V. Dnana. Who could you hire as a valid TD ? Only M. Dualliac could manage this...
I propose replacing Wei-Hwa's (or should I say Huang's?) perpetual kcehc rule with the rule that if a player may retract so as to create a position which has occured at least twice before, with the same player to retract, and the same set of retractions legal, then he may claim a draw by 3-fold repetition.
I think that a player can prevent his opponent reaching the array in any finite number of moves, merely by blockading his opponent's men's starting squares. Rules such as Wei-Hwa's "A player loses if after fifty of his moves he has not uncaptured any of his opponent's pieces, and not vacated any of his opponent's starting squares." are artificial and don't do enough to prevent this blockading tactic.
Perhaps it would be better to play Hoeg-retractor. That is, play goes in the following sequence:
- Black retracts a move, - White optionally adds a white man whose capture has been retracted, - White retracts a move, - Black optionally adds a black man whose capture has been retracted, provided that at all times the retractions are legal.
I prefer to ditch the aim of reinstating the array, and go for Philippe's aim of being in a position to retract a single-move and then mate in 1. RA is still very much involved, as a player may still challenge the legality of either his opponent's last retraction or the choice of uncaptured man.
You lose Philippe's tactic of retracting Kb2xBa1, but you gain a far greater freedom to get your men back. I haven't tried it, but perhaps there's a risk of brutal mates with lots of queens on the board. In this case, make an additional rule that each retraction must be consistent with there never having been any promotions. So each player may not add more than one queen.