"Pawns may not be promoted to bishops;"
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For real, I would absolutely love to know why that constraint was needed. Does anyone have info on why?
??????????
For real, I would absolutely love to know why that constraint was needed. Does anyone have info on why?
Amusingly enough Rybka, the top engine for many years comparatively recently didn't implement bishop underpromotion either. For a much different reason. The programmer chose a not so great low level bit representation convention that forbid representing this case. He refused to fix the error because it would be too much work for the more-or-less non-existent situations where promoting to a bishop is the best move.
For an extreme example, see https://en.wikipedia.org/wiki/Babson_task.
Knight 1e-3 Rook 1e-5 Bishop 1e-7
A complicating factor in measuring this is that often a rook serves just as well as a queen, the opponent is going to capture immediately anyway, so players sometimes underpromote to a rook just for the fun factor. My number is more meant to represent cases where underpromotion really is the best move. Knight underpromotion is by far the most important, because a knight can do things a Queen can't, most typically give check to keep a combination going without giving the opponent an opportunity to play an active move. Rook and Bishop underpromotions can only be advantageous when they simultaneously avoid stalemate and keep a winning advantage. This is much more likely with a rook, for example in the famous Saavedra position, perhaps the most famous chess problem of all (it featured as decoration on our wedding cake :).
Edit to add: Here's the game. It was Caruana vs Robson 2021 and occurred in the 7th round of the USCF championship. Fabi's final move was to promote to bishop at which point Ray Robson resigned https://lichess.org/study/embed/iLDop9iy/wCdsPJJI He did it for style points- a queen would have been just fine in that position.
If you only remove the bishops from the starting position, then the rules of chess still allow bishops to come in later in the game (through a promotion).
I am assuming they did not have enough memory to hold the rules of bishops, and therefore decided to completely remove them from the set of possible pieces. They allow rooks and knights promotion because these pieces are in the set of initial pieces: their rules are already implemented.