Kgr?: This is a life or death question, I suppose. While playing against a computer (GnuGo) it abandoned a structure that I can't figure out why it's dead. I've tried a whole bunch of variations which all end in white (the computer) making some kind of life in the corner, and I haven't come up with any where it dies completely.
At first I thought it might be a bug, but I can give the structure a lot harder of a time than I realized, so now I'm wondering if there's some tesuji that I have missed.
Here is the position where the exchange started (this is somewhat late in the endgame, or so I thought):
Everything is obviously alive except (maybe) the white structure in the upper left, I left out the rest of the board which is all completely settled and seemed irrelevent.
Next came the following sequence, culminating in (4) with what I thought was just maybe a good reducing move.
After this, white didn't respond in the corner to anything I did (5 was tenuki). I tried various 6s using the undo button since I was just so incredulous, but it didn't respond to anything.
I tried a bunch of variations where I kill everything except something it builds in the corner, but I couldn't find any variation where I killed it all.
Can anyone see what I'm missing to explain white's behavior?
(I hope I've done this all right, this is the first page I've created here)
KarlKnechtel: GnuGo isn't perfect, so that could be your answer. It's also possible that it figured the corner was alive even with another local move by B, and therefore treated B 4 as gote. It looks to me like the whole thing is going to end up connected, so as long as it makes two eyes, B 4 is just however many points worth of reduction.
Note that without the marked white stone, black could jump one space further (see gorilla).
HolIgor: GnuGo does not see a threat. This happens often with computer programs. They can't read positions. Perhaps the group satisfied some criterion like 5 liberties for tactical stability, so GnuGo did not even consider a possibility of death here.
Jesse: I checked what GnuGo was "thinking" using decide-dragon.
It thinks that the white group is already dead, since it only looks at defending with a, b, or c. After is added it can tell that white will live.
Kgr?: Thanks everyone. I did forget to mention that the tenuki it played on 5 was a 2 point capture in gote, and aside from this corner there was nothing bigger than 2 points on the whole board. But it does seem clear from what Jesse said that it was a mistake and not some deep reading.
Jesse, I had no idea one could do that at all; it is a neat feature to play with (and I wouldn't have needed to ask here). For those who don't know I found this page, which describes all sorts of analysis of its actions that gnugo can do: