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MatPlus.Net Forum Retro/Math Kofman's castling idea with en passant
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(1) Posted by Siegfried Hornecker [Sunday, Sep 23, 2018 22:39]; edited by Siegfried Hornecker [18-09-23]

Kofman's castling idea with en passant

There is a famous Kofman problem where two castlings are mutually exclusive. White retracts castling to plays it, as otherwise he couldn't prove that castling is legal. Yea, this is obsolete now, but whatever. Enjoy!

(= 5+2 )

SH, Original
White retracts a move, then checkmates in one move

Solution: White retracts 1.h:g6 e.p., then plays 1.B:g5 mate. He can't play 1.h:g6 e.p., as there is no proof the last move was the double-step.
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(2) Posted by Joost de Heer [Monday, Sep 24, 2018 15:48]; edited by Joost de Heer [18-09-24]

Unfortunately, that's not how retractors work. If you retract a move, you can play that move in the forward play. In Kofman's case, the retraction and forward play purely is played to ensure the invalidity of the black castling, because actually playing the castling move is the only way in that position to prove that white did castle.

IIRC there's a fairy condition 'Amnesia' which causes the loss of game history after a move, and with that fairy condition, your composition would be correct, because white indeed can't capture ep after retracting hxg6, because he can't prove that g7-g5 was the last move.

See also the Champagne tourney from this year, Marko Klasinc' second prize in category B.
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(3) Posted by Siegfried Hornecker [Monday, Sep 24, 2018 18:56]

That's what I meant when I said, it's obsolete now.
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MatPlus.Net Forum Retro/Math Kofman's castling idea with en passant