I'll check it out if my current plan fails

Current plan (assuming White to move)
============

After checking for stalemate and checkmate, get a move (e2-e4) from DisplayBoard. Board unchanged yet

(assume opponent pawn at d3 I don't know about yet)

Find LegalMove("e2-e4") is legal

If piece at BOARD-e2 is an unmoved K or R, change it to "moved". Remove piece in BOARD-e2 and put it into BOARD-e4. (obviously I really only need to test a1-a8)

Check BOARD-d3 and BOARD-f3. If there are Black pawns there, change them to "pawn can be taken en passent"

Check other positions. If there are White pawns that are marked "pawn can be taken en passent", change them to regular pawns. White missed its chance

Mac