Tries: 1.Kc4? but 1…exf1=Q+! 1.Bxe2? but 1…Sd6! Key: 1.Kc3! (thr. 2.Bb4+ Kxa4 3.b3#) Sxc5 2.Rxc5+ Kxa4 3.b3+ Ka3 4.Ra5# 1...Kxa4 2.b3+Kb5 3.Bxe2+ Ka5 4.Bb4# 1...e1=Q+ 2.Kb3 (thr. 3.Bb4#) Qxe3+ 3.Bxe3+ Sc5+ 4.Rxc5# 3...c5 4.Bd2# 2...Qc3+ 3.bxc3 (thr. 4.Bb4#) Sd8/d6 4.Bb4# 3...Sxc5+ 4.Rxc5#
A fourmover with promotion check provocation. The key threatens to open the battery with effect, giving black the opportunity to promote with check or promote by capturing white’s bishop. In the problem, the try 1.Bxe2? Sd6! is important because the refutation involves delaying white only long enough with a check - from the weakest piece on the board (and white can capture it with check!), the S, whereas the key invites a promotion check from the strongest piece on the board, a queen, (which cannot be captured straight away!).
