Refusing quadrupal ko
This is a key game from yesterday's Samsung Cup preliminary tournament between Na Hyun 3-dan and Shi Yue 9-dan.
At this moment, the position was extremely close. The upper-left corner has a double ko and black is unconditionally alive. Let's have a look how Shi Yue exploited this situation.
and are all forcing moves. Then is a brilliant tesuji to build a ko shape in the corner.
After white can throw in at 'a' to start another ko. Without the first double ko, this would be hopeless for white. But now white has unlimited ko threats so his group cannot be killed. This situation would be equivalent to a quadrupal ko to void this game. However, Shi Yue had a better plan ......
Omitting some exchanges at the upper-left corner (Shi Yue used the double ko to gain extra thinking time in byoyomi), white played forcing move and before playing to form an approach ko (black has no time to capture from the side of 'a'). White needs two extra moves - 'b' and - to make it a direct ko. This is normally hopeless. But because of the upper-left-corner double ko, black had to waste a move at to eliminate the ko threats there. So this is in fact a one-step approach ko. And white happened to have just enough ko threat to win it.
Ignoring , Shi Yue played to create a direct ko. And happened to be a suitable ko threat. So Na Hyun had to resign.