"King, Queen, Knave": Solution
Solution to King, Queen, Knave, from last Sunday:
White's last move must have been an underpromotion, with a pawn on d7 capturing a knight on c8. If we retract that move we get the position above, and if the pawn instead now captures the rook on e8 and promotes to a knight, it gives mate. This is the only legal possibility that meets the requirements of the problem.
Nabokov writes, "There is some mild magic in the retrospective transformation of White R into Black Kt, and Black R into White Kt, with the symmetry of the pieces (and White's defense of c7) retained."
