Tsumego From Games 4
This comes from a three-stone game with me (1k EGF) playing White.
The attention is drawn to the upper right corner. The White group is trying to live thus cutting the top center group from the upper right corner. Had I calculated it a bit earlier, I'd have left this as aji. Black can kill this group.
My opponent played 1 and 3. If I continued with a, Black b would still have killed me. If White b, then Black c kills. But, there was a tesuji to save the white group.
If Black connects, White lives with 5. White 5 at a would be a big mistake, for Black b will make miai of 5 and c to kill.
This seems to be a ko -- if white connects with 4 at 1, black at a makes a double damezumari. Thus, white 4 must be at a and black starts the ko.
unkx80: Unfortunately your argument doesn't hold water. lives.
JoelR: This comment does not make sense. is self-atari, White captures (more than) three with to make an eye. After White fills , there is no ko in this area.
(inoru?:) Doing so forms this shape, and white is successfully alive.
JoelR: On the other hand, if White plays here, then Black can recapture the ko at , and White is not assured life.