Retrograde Analysis Problem 2
This is Hane Yasumasa (w) - Ma Xiaochun, 1990. After 54 moves, 21 out of 27 white stones are still on the board and 23 out of 27 black stones. It is quite obvious what happened to the six white stones and to three of the four black stones, but what about the fourth black stone? Please mark the point where you think it was played!