Single Comment

To H.G.M.: Harm, you are proposing a lot of things simultaneously. So let me answer slowly. There should be a main primary demand, that such an extended FEN should end in an IDENTICAL string, when applied to common chess variants. Otherwise I would not like to support it. There already has been a quarrel concerning X-FEN and Shredder-FEN, which is incompatible to that demand. Nevertheless in the early days of X-FEN I modified its definition concerning a less important element to also cover the proposed additional demand to have a partial castling string in FEN NEVER longer than 4 letters. I would like to preserve that state of the art of X-FEN.

Actually I would like as a first step to merely support castling rights, where the castling piece will be placed upon the next inner square neighbored to the (one only) castled King. The King will be placed by default THREE steps from the a-side performing 'O-O-O' and TWO steps from the other side performing 'O-O'. The algebraic notation should use source and target coordinate, when the king is moving at least two steps when castling with the most outside Rook. Otherwise the coordinate of the involved piece has to replace the (redundant) target square. This is for to serve two goals: a) to avoid a mixing up with traditional King's moves, b) to give the GUI a unique information, by which input gesture a castling should be intended without any doubt. Such a modification should not be interpreted as a 'capturing' of the involved piece, instead as a jostling it for to have it join the castling procedure. 

To encode the castling rights: the default is to address the most outer Rook by traditional letters 'Qq' for the a-side, 'Kk' for the opposite side. In any other case the column letter of the involved piece should be used: in upper case, where a white colored piece is addressed, in lower case, where that piece would be black colored.

Now to different castling methods: because such methods would target all possible castlings (never different methods for different pieces) it does not make sense to encode this related to the elementary castlings. Instead the whole partial castling string inside a FEN has to be addressed. In SMIRF I solve this by applying a special prefix letter, which has to be different from any possible column letter to avoid misinterpretations.

Now let me stop at this point, to have the written text being discussed.