Originally posted by Mephisto2
Great job! Not only to find the solution, but also the explanation is quite rigorous!
Perhaps one extra clarification 1. 0-0-0 allows white to claim the 50-moves rule, as was clearly shown above. If the starting position had occurred via an other way that involved white losing his castling rights, then it can be shown easily that the position could have ...[text shortened]... ince the last capture/pawn move (the rather complicated shuffling with the rooks is not needed).
Also, since WK has been on e1 all game, the Bg3 cannot escape via f2. If he could, then we could start by retracting 1...Bh2 2...Bg1 and White has h2-h3 at his convenience to kill the 50-move count.
