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Well, Harm, I intend to seperate the variant depending renaming of piece
types from FEN, because this should be a feature of the GUI and maybe
become customizable at least. It might be accompanied by a commenting PGN
token like: [Variant='Janus Chess, J:=A']. That would give to it an
importance like a kind of comment. But the X-FEN as a communication
vehicle for engines should use merely unified and thus constant piece
letters. This should also be true for move notations (in PGN / Algebraic),
where protocols e.g. like UCI demand for one-char-letter piece symbols.

Of course this approach is not at all able to cover all piece types. But
that goal would need a much bigger approach than what I intend this
moment. (Octopus would not be able to cover every piece type, too.) And a
later extension e.g. done as proposed in your posting would not at all be
made impossible by my idea. But it would be a simple and compatible ad hoc
extension into the right direction.

Promoting possibilities should not differ through different incarnations
of the same piece type. Thus having pawns with different promotion
abilities does not make sense to me. Thus it has to be related to a given
board situation in total.