
Resulting position: White to play and mate in 9 by White.
Bg3 Bg1
Na7 Bc7 b5 a6
Bg3 Bg1
Unfortunately, the black bishop can't legally (retro analysis) be on g1 on White's turn. This double check wouldn't work.
If you meant the opposite, which implies Bh4 moving to g3 with check as the last move leading up to this suggested position, then after 1.Kxg3 there is no followup for mate in one by Black. Which means it is also incorrect.
Na7 Bc7 b5 a6
According to the correct piece placement of all the other pieces, I understand you solved it (good job!) and I know you understand where to place the black knight. There was only a small transcription error on your part since there already is a black knight on a7.
Splendid! That was indeed the reasoning
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