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MatPlus.Net Forum Retro/Math Fairy legality uncertainty
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(1) Posted by Juraj Lörinc [Thursday, Dec 22, 2016 18:44]

Fairy legality uncertainty

Usually, legality of position is not considered in fairy problems, they are normally considered as start from diagram position. (Otherwise, e.g., it would be illegal to have PWC problems with other than 16+16 pieces on the board.)

The situation is different in a case of fairy problems that are based on retro play. Anticirce Proca retractors are a good example. My understanding is, that if there is some retro play involved, especially when proving e.p. legality in the key, the position should be legal under given fairy condition.

(If any of my assumptions given until now is wrong, just please, correct me.)

And the diagram I am not sure about:

Dieter Werner & Norbert Geissler
3rd Prize 1st Fairy TT Orbit 2014
(= 7+8 )
Alphabetic chess

The author's intention includes try 1.d5?? that is illegal as it can be proven under Alphabetic chess rules that the last move must have been 0...c2-c4!, therefore Black must start with 1.bxc3 e.p.! and then proceeds with AUW and castling, rounding good quality Valladao.

However I wonder if the whole position is legal under Alphabetic chess rules at all. My tries to play from the game starting position were quite unsuccessful, leading always to quite destroyed queen's flank. Can anybody find a proof it is legal or it is illegal?

And if the position as whole is illegal - can we argue that last move must have been 0...c2-c4!?
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(2) Posted by Geoff Foster [Thursday, Dec 22, 2016 22:26]

For fairy retros, it is too strict to require that the position be legal under the fairy condition. For AlphabeticChess the position would almost never be legal. Perhaps the position should be legal in a *normal* game of chess up to the beginning of the retro play. In this case the position (with wPc4 on c2) was reached under normal rules, then AlphabeticChess "switched on" and White played c2-c4.
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(3) Posted by Georgy Evseev [Friday, Dec 23, 2016 08:40]

When have they switched on?

If "just" then the last move could have been Ka1-a2.
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(4) Posted by Siegfried Hornecker [Friday, Dec 23, 2016 09:31]; edited by Siegfried Hornecker [16-12-23]

I think it is assumed the last moves was c2-c4 and at some earlier point we had c3:b2. But what is taken on b2? Somehow White must have disallowed the Black a-pawn to move earlier, so the c-pawn moved. But what blocked it, and how was it captured? Maybe that is our piece on b2, so that would be a knight. So b2 must come from a3 to capture after Sa4-b2, meaning that Pc7 captured to the a-file and then to b2.
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MatPlus.Net Forum Retro/Math Fairy legality uncertainty