Helpmate in 18, White king check avoidance

Another helpmate for fun. Before the dark-square bishop can sacrifice itself at c7, W's king needs to be sheltered from check by the black rook. This means the light-square bishop has to come over to f1.

The whole timing and strategy, as Black's king takes just the pawns on b5 and b6 and gets checkmated on a4, by the newly promoted queen coming from c8 to a6, has strategic appeal that is classic helpmate more-mover.

Note the elegant timing of the released black pawn on b7, creating the self-block on b4 in the nick of time.

This was part of a nice article from Ljubomir Ugren (Zagreb) and Marko Klasinc -Ljubljana entitled "Rekordi (tasks) Pomocnog mata" in the April 1973 issue of Problem.