I came across this position when playing with a position from a random pro game and I am stumped why the computer is claiming one move is a -1.5 or more error. Normally one would play as in the diagram, preserving good aji and avoiding to play the one-sided block on the second line.
- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . X . . . . . X . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . X X X . . |
$$ | . . . . . . . . . . . . . O O O . . . |
$$ | . . . O . . . . . , . . O . . , O O . |
$$ | . . . . . . . . 1 . . X X X X O O . O |
$$ | . . . . . . . . . . . . . O O X X O . |
$$ | . . . . . . . . . . . . . . . . . X . |
$$ ---------------------------------------[/go]
What if you play the block? Does it matter objectively in this position and how much does it matter?
- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . . . X . . . . . X . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . X X X . . |
$$ | . . . . . . . . . . . . . O O O . . . |
$$ | . . . O . . . . . , . . O . . , O O . |
$$ | . . . . . . . . 3 . . X X X X O O . O |
$$ | . . . . . . . . . . . . 1 O O X X O . |
$$ | . . . . . . . . . . . . . . . 2 . X . |
$$ ---------------------------------------[/go]
Initially my b40 katago setup thinks this is almost a 2 pt. error but adjusts down to 1.6 after a lot of playouts. Besides being a move that black doesn't seem to need to play and you shouldn't, I don't understand how the computer concludes it is this big of a mistake and why in the range 1.5 - 2.0 points. It is also a large drop in percentages. Maybe someone understands these things?
I have some theories:
1. There is a large ko threat, maybe the only good threat in the position, but then there is no ko anywhere anyway.
- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . X X X . . |
$$ . . . . . . O O O . . . |
$$ . . , . . O . . , O O . |
$$ . X . . X X X X O O 3 O |
$$ . . . . . . O O X X O 1 |
$$ . . . . . . . . . . X . |
$$ ---------------------------------------[/go]
2. Depending on how you play it there are many small, even endgame, ko threats. Maybe 1.5 pts. is a lot for just these threats, and there are still some threats anyway.
- Click Here To Show Diagram Code
[go]$$c
$$ ---------------------------------------
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . X X X . . |
$$ . . . . . . O O O . . . |
$$ . . , . . O . . , O O . |
$$ . X . . X X X X O O . O |
$$ . . . . . 3 O O X X O 2 |
$$ . . . . . . 5 . 6 1 X 4 |
$$ ---------------------------------------[/go]
- Click Here To Show Diagram Code
[go]$$cm7
$$ ---------------------------------------
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . . . . . . |
$$ . . . . . . . X X X . . |
$$ . . . . . . O O O . . . |
$$ . . , . . O . . , O O . |
$$ . X . . X X X X O O . O |
$$ . . . . . X O O 1 2 O O |
$$ . . . . . . X . O . . O |
$$ ---------------------------------------[/go]
3. I had the idea that the computer was wrong and the presence of some extra stones was throwing it off. In this case it should correct itself, on the theory that it is usually right and when you add more moves it will end up in a position were it is right again. So I tried to play a few moves from the top line and then run a lot of playouts but it never changed its' mind and curiously continues the game mostly the same way compared to no bad second line block.
4. I just missed something and the bad block is bad for some other reasons
![Confused :-?](./images/smilies/icon_e_confused.gif)