I would suggest that by analogy with Appendix A4d -Roger de Coverly wrote:Would that cause a loss under a strict interpretation of the new FIDE rules for rapidplay? It's obvious there's been an illegal move, but not one that's been immediately spotted and the game claimed.Alex McFarlane wrote: The opponent then realises he is facing 2 black squared bishops and assumes it is the last move which was illegal.
- the correct decision would be for the arbiter to consult with the players and if one claims the position occurred on the previous move and the other that it had been there for some time then if it is not possible that the extra same colour bishop was the result of a promotion (still 8 pawns on the board for that player) the arbiter should declare the game drawn. Otherwise he should tell the players to play on.If the arbiter observes both kings are in check, or a pawn on the rank furthest from its starting position, he shall wait until the next move is completed. Then, if the illegal position is still on the board, he shall declare the game drawn
Don't forget this part of A4b -
If the arbiter does not intervene, the opponent is entitled to claim a win, provided the opponent has not made his next move.
If it cannot be established that the illegal position arose on the just completed move then the subsequent move by the claimant invalidates the claim.
It is also not clear that the position arose as the result of an illegal move. It could be, say, that one player played Bc1-g4.5 (half on g4, half on g5) and the bishop was later accidentally adjusted by one of the players to g4 instead of g5.