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Note that all of Black's moves are forced:
- Black king cannot escape on a7 because of the white b6 pawn;
- Black queen cannot parry an a-file check on a7 because it would actually be a suicide;
- Black king cannot escape on c8 .
If 13... Kb7 then 14. Qa6#
If 13... Qa7 then 14. Qxa7#