After , White is left without two eyes.
threatens to capture the three white stones to the left of . White can not form two eyes without these stones. White must connect these stones to the rest of the White group using a or prevent Black from capturing these stones by playing , which makes a Black attempt to capture the stones by playing at a a self-atari.
Unfortunately for White, neither White a nor make life. threatens to make a second eye with , but prevents that.
The key to the problem is the first rule of killing groups: Examine a play at the eye-point.
Brent: For a while I had convinced myself that worked as well, but then realized that it doesn't, so I thought I'd add a failure diagram to show why. After White cannot connect at a because of shortage of liberties. However, solves White's problems. White threatens to now connect at a and live, so...
...Black must capture there instead. But now White plays and lives: playing at b in an attempt to falsily an eye would be auto-atari for Black. So Black must connect out first, giving White the chance to play at b and solidify the second eye.
unkx80: doesn't solve the problem, kills. After which, a and b are miai.
This also works. Therefore, by "problem principles" alone, one can deduce that the correct solution is at the other key point. ilan
Unfortunately, this does not work either. --unkx80
Ooops, forgot about the right hand side. I die an ignominious death. ilan