Here's a review using LeelaZero of Andy Liu's impressive win against Ahn Dalhoon 9p at the US Go Congress.



Winrate graph:


Edit: added variations for black answering press at 13.

Thanks for the review, I have a pair of questions:

1) One question I always have when reviewing what look like simple pro mistakes is whether they're as simple as they appear. At , is there anything that favors the solid connection? I couldn't see any advantage to it.
2) After the R5 approach, and the variation with E14, E15, E13, what is the meaning of the C13 tesuji?

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Occupy Babel!

1) It is more solid in the corner so white has fewer ko threats with d18 b17 etc. Also with d18 and c18 as atari white has one more liberty so if he ever went into the top side with h17/18 then Black needs to be careful with his liberties. White needs to be careful of e14 too.
2) My feeling is it is a timing issue: it's a shape weakness of black but if you hit it later once black has more stones in the left side from chasing white then he may be able to resist and gobble you up whole instead of needing to answer submissively, in which case they become some useful aji for white. I'll try to tease an explanation from LZ.

1) Here is an example sequence of exploiting weakness of b15 in the corner. White 2 takes care of e15 and is a a key point to press black low and keep white's group strong. Black 3 tenukis because c13 block is not that big with b15 there (locally d14 push, e14 answer and e12 peep is good but lower corners are bigger). White 4 hanes in corner and black 5 cut is a mistake (should be c18 clamp but it's annoying to have to concede points/shape like this) but let's see why. 6 is atari (could actually keep this in reserve) and now 8 is sente (if tenuki then e19 captures). Black's best choice is to submit with 9-a but this is painful. Note that j17 cut doesn't work as due to ladder thanks to m15.

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . a . . . . . . . . . . . . |
$$ | . . 6 4 X X . 9 0 . . . . . . . . . . |
$$ | . 7 X 5 O X X 8 . . . X X . O . . . . |
$$ | . X O O O O X O . , . X . X . , X O . |
$$ | . 1 . . . X O O . . . O X . . O O . . |
$$ | . . X . . O . . . . . . . . . . . . . |
$$ | . . . 2 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . 3 . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

And even if the ladder didn't work white could extend and net as black owes a move inside (can't play b18 at k17 or lose by 1 liberty):

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . 2 . . . . . . . . . . . |
$$ | . 5 O O X X 3 X O 4 . . . . . . . . . |
$$ | . X X X O X X O 1 . . X X . O . . . . |
$$ | . X O O O O X O . 6 . X . X . , X O . |
$$ | . X . . . X O O . . . . X . . O O . . |
$$ | . . X . . O . . . . . . . . . . . . . |
$$ | . . . O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

None of these sequences would work with the solid b15 connection because white can't get a move at c18 with the b18 liberty open for free. It would instead look like this and h17 is just totally gote and therefore small.

Click Here To Show Diagram Code

[go]$$Bc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . 9 8 6 X X . . . . . . . . . . . . . |
$$ | . 1 X 7 O X X 0 . . . X X . O . . . . |
$$ | . X O O O O X O . , . X . X . , X O . |
$$ | . . . . . X O O . . . O X . . O O . . |
$$ | . . X 3 4 O . . . . . . . . . . . . . |
$$ | . . 5 2 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

2) If white directly jumps here's an illustrative sequence (not best moves for both but not more than a few % minus).

Click Here To Show Diagram Code

[go]$$Wc
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X . . . . . . . . . . . . . |
$$ | . X X . O X X . . . . X X . O . . . . |
$$ | . X O O O O X O . , . X . X . , X O . |
$$ | . . . . O . O O . . . O X . . O O . . |
$$ | . . X . X O . . . . . . . . . . . . . |
$$ | . . . . X . . . . . . . . . . . . . . |
$$ | . . . . . . 1 . . . . . . . . . . . . |
$$ | . . . . . 4 5 . . . . . . . . . . . . |
$$ | . . . , . 6 7 . . , . . . . . , . . . |
$$ | . . . . . 8 . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . 2 . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . 3 . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Click Here To Show Diagram Code

[go]$$Wcm9
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . X X . . . . . . . . . . . . . |
$$ | . X X . O X X . . . . X X . O . . . . |
$$ | . X O O O O X O . , . X . X . , X O . |
$$ | . . . . O . O O . . . O X . . O O . . |
$$ | . . X . X O . . . . . . . . . . . . . |
$$ | . . . . X . . . . . . . . . . . . . . |
$$ | . . . . . . O . . . . . . . . . . . . |
$$ | . . . . . X O . . . . . . . . . . . . |
$$ | . . . , . X O . . , . . . . . , . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . 6 . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . 5 . O . . . . . , . . . 3 1 , . . . |
$$ | . . . . . O . . . . . . 4 . 2 X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

After black has defended at e6 the c13 attachment became much less attractive, whereas LZ wanted to play it as white earlier in this sequence. Now if white does it black can answer in a tougher way and it's hard for white to get anything in the left side.

This probe timing reminds me of this attachment in the micro-Chinese opening, which you want to play before pushing at a (because if you do it afterwards they are strong and answer at c instead of b to leave you with less good future possibilities).

Click Here To Show Diagram Code

[go]$$B
$$ +---------------------------------------+
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . a O . . |
$$ | . . O . . . . . . . . . . . . 1 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , X . . |
$$ | . . . . . X . . . . . X . . . b 2 c . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ +---------------------------------------+[/go]

Thanks, I should've said that I saw that there's a difference of ko threats, but I'd ordinarily think that isn't enough to explain why a pro made a mistake like that.

_________________
Occupy Babel!