81. cannon and screen together, by the cannon

Helpmates in xiangqi

4 moves

Black moves first and both sides cooperate: Black, Red, Black, Red, and Red's second move mates. Black has 21 legal first moves and exactly one of them works. 8 pieces, Black 12.5 to Red 8.5 in material, mate by the cannon. Themes: cannon and screen together, Black blocks its own…