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H. G. Muller wrote on Fri, Dec 23, 2022 07:03 PM UTC in reply to Greg Strong from 06:39 PM:

Well, having board other than 8x8, or piece IDs A or C, is also not compatible with the FEN spec. One has to do some amount of generalization to make it useful for variants. The 'castling field' in fact already is a field that indicates virginity of pieces. But only of those for which it matters, because under orthodox rules only castling is affected by virginity. (One could argue about Pawns, but the double-push is usually explained as a location-depended move rather than an initial one, as evidenced by the rules of Crazyhouse/Bughouse.)

I think it would be bad to have two fields that basically serve the same purpose. In Shredder-FEN notation the letters in this field already identify the piece, so there is no need to make separate fields for Kings+Rooks, Knights, Bishops and Queens. It seems much better to have a unified virginity field.

Likewise, the the e.p. field indicates the last-moved piece, in positions where that matters. So it would be logical to use it for any other type of last-moved piece as well.

I agree that leaving in the right of a non-existing piece is perhaps a bit contrived. But how about the other option: indicating an undetermined right by X? Then you would not have to look at the board. An X in the field means both Bishops are virgin and can convert. One gets captured without moving: the X remains, to indicate all remaining Bishops still have choice. If one moves, the rights disappear altogether when it converts, and change to indicate the file of the Bishop that now must convert.

If you have both converting Camels and Bishops, just use X for the Bishops and Y for the Camels. (Or more generally, adopt the convention that X indicates the outer-most piece type, working your way inwards when stepping through the alphabet.)


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