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Thank you, Harm, for your explanations. Concerning X-FEN I would prefer to
have piece letters being unique and compatible to nearly established
Archbishop and Chancellor. I do not want to change used letters with the
variants playing them.

Smirf unfortunately cannot be extended to handle all those four super
pieces because of its piece codes' bit encoding their properies. But in
Octopus there will be a more flexible bit encoding, so that a lot more of
gait combinations might be possible in Octopus, e.g.: Q+N, K+N, K+B, K+R.

When I read Superchess documents correctly, a promotion to Q+N is not
permitted. The union of all usable pieces seems to be constant, thus it
might be unnecessary to have the unused pieces listed seperately inside of
an X-FEN. Nevertheless it is not obvious, that it would be a notation from
Superchess. To have a unique X-FEN method I intend to do following:
instead of a []-list of captured pieces (which I am not yet able to
support, but maybe later ...) there could be an optional separated tag '
:' followed directly by forbidden pieces' symbols (if any). By default
at 8x8 NBRQ and at 10x8 NBRACQ are promotable pieces. For e.g. Janus Chess
thus ' :C' would symbolize, that a Chancellor would not be allowed to be
selected for promotion in this situation. Extending this to Superchess'
piece set there could be a representing symbol '*'. So ':*G' could
symbolize that any available castling piece has to be selected within the
unused super pieces set and that the (G = Giant) Q+N piece is not allowed
to be promoted in.