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 Post subject: Why did he play this move?
Post #1 Posted: Tue Jul 30, 2019 5:05 am 
Lives in sente

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Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . X . . O . . . . . . . . . . . X X . |
$$ | O X X O . . . O . . . . . . . O X . X |
$$ | . O X X O O . O X X O O O . O O O X . |
$$ | O O O O X O . . . , . X X O O O X X O |
$$ | a O X X X O X . . X . X O . O X X O X |
$$ | X X X . . O X . . . . X O O X O O O . |
$$ | . X . . . O X . . X X O O X X X O . . |
$$ | . O O X . O X . . . O O X X O . X O . |
$$ | . . . . . O X . . . . . X O . . X O . |
$$ | . . O , O . . . . , O . O X . , X O . |
$$ | . . X X . . X . . . . . . . . . X O . |
$$ | . . . . . O O . . . . . . . . . . O . |
$$ | . . . . . . . . . . . . . . X . X X . |
$$ | . . X X . . . . . . . . . O X . . . . |
$$ | . . O . . X . . . . . . . X . . . . . |
$$ | . . . O . . . . . O . . . X O O O . . |
$$ | . . . . . O . . . . . . O O X X X O . |
$$ | . . . . . . . . . . . . O X . X . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ ---------------------------------------[/go]


Xu Jiayang 8p - Yao Xiaotong 2p, W+4.5 (Komi 7.5)
21st Chinese League A, round 10, 2019-07-22


Black to play (He chose move a)

It seems to be a clear mistake. What was he thinking? I can not find the motivation for the move.

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 Post subject: Re: Why did he play this move?
Post #2 Posted: Tue Jul 30, 2019 5:24 am 
Dies in gote

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To eliminate the C19 ko?


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 Post subject: Re: Why did he play this move?
Post #3 Posted: Tue Jul 30, 2019 5:29 am 
Lives in sente

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but the ko does not seem like a real threat?

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 Post subject: Re: Why did he play this move?
Post #4 Posted: Tue Jul 30, 2019 5:43 am 
Lives in sente

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Ok, thanks now I can see the motivation.

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 Post subject: Re: Why did he play this move?
Post #5 Posted: Tue Jul 30, 2019 6:15 am 
Dies in gote

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Unclear exactly what ko threats he was afraid of, but seems he just wanted to kill the aji.

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 Post subject: Re: Why did he play this move?
Post #6 Posted: Tue Jul 30, 2019 8:29 am 
Lives with ko

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Black feels far enough ahead to take the slow move and secure the victory. He has a lot of solid points like both corners on the right hand side. He doesn't want the risk of losing a 40+ pt ko, (c18 dies and the c14 group does not have assured life) or allowing his opponent to profit from threatening. White has a large wing on the bottom of the board but black can easily invade it at many points.

Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . X . . O . . . . . . . . . . . X X . |
$$ | O X X O . . . O . . . . . . . O X . X |
$$ | . O X X O O . O X X O O O . O O O X . |
$$ | O O O O X O . . . , . X X O O O X X O |
$$ | . O X X X O X . . X . X O . O X X O X |
$$ | X X X . . O X . . . . X O O X O O O . |
$$ | . X . . . O X . . X X O O X X X O . . |
$$ | . O O X . O X . . . O O X X O . X O . |
$$ | . . . . . O X . . . . . X O . . X O . |
$$ | . . O , O . . . . , O . O X . , X O . |
$$ | . . X X . . X . . . . . . . . . X O . |
$$ | . . . . . O O . . . . . . . . . . O . |
$$ | . . . . . . . . . . . . . . X . X X . |
$$ | . . X X . . . . . . . . . O X . . . . |
$$ | . . O . . X . . . . . . . X . . . . . |
$$ | . . . O . . . . d O . . . X O O O . . |
$$ | . . a . . O . b . c . . O O X X X O . |
$$ | . . . e . . . . . . . . O X . X . X . |
$$ | . . . . . . . . . . . . . . X . . . . |
$$ ---------------------------------------[/go]


Basically, once black secures all three of the other corners, white has too much aji to hold all the bottom, and thus the game is over for white. Even invading here isn't needed, so black probably aims to protect his D9 stones.

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 Post subject: Re: Why did he play this move?
Post #7 Posted: Tue Jul 30, 2019 8:41 am 
Judan

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Hades12 wrote:
Black feels far enough ahead to take the slow move and secure the victory ....
Basically, once black secures all three of the other corners, white has too much aji to hold all the bottom, and thus the game is over for white. Even invading here isn't needed, so black probably aims to protect his D9 stones.

but:
Quote:
Xu Jiayang 8p - Yao Xiaotong 2p, W+4.5

Of course black might be winning now and messed up later.

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 Post subject: Re: Why did he play this move?
Post #8 Posted: Tue Jul 30, 2019 9:30 am 
Lives with ko

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Uberdude wrote:
Hades12 wrote:
Black feels far enough ahead to take the slow move and secure the victory ....
Basically, once black secures all three of the other corners, white has too much aji to hold all the bottom, and thus the game is over for white. Even invading here isn't needed, so black probably aims to protect his D9 stones.

but:
Quote:
Xu Jiayang 8p - Yao Xiaotong 2p, W+4.5

Of course black might be winning now and messed up later.


Our feelings aren't always right :(

would you prefer white Uber? Just observing, I like black. I might have to look up this game.

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 Post subject: Re: Why did he play this move?
Post #9 Posted: Tue Jul 30, 2019 12:10 pm 
Judan

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I can't tell who is winning from a glance: I'd guess that this game is closer than the margin of error in my counting for such a middle-game position, but I'll give it a go assuming black eliminates the ko and adding a few simplifying sente yoses:

Click Here To Show Diagram Code
[go]$$Bc
$$ ---------------------------------------
$$ | . X . . O . . . . . . . . . . . X X . |
$$ | O X X O . . . O . . . . . . . O X . X |
$$ | . O X X O O . O X X O O O . O O O X B |
$$ | O O O O X O . c . , a X X O O O X X . |
$$ | 1 O X X X O X . d X b X O . O X X O X |
$$ | X X X . . O X . . . . X O O X O O O W |
$$ | . X . . . O X . . X X O O X X X O . . |
$$ | . O O X . O X . f . O O X X O . X O . |
$$ | . . . . . O X . e . . W X . B . X O . |
$$ | . . O , O . . . . , O . O X . , X O . |
$$ | . . X X . . X . . . . . . . . . X O . |
$$ | . . . . . O O . . . . . . . . . . O W |
$$ | . . . . . . . . . . . . . . X . X X W |
$$ | B B X X . . . . . . . . . O X . . B B |
$$ | B W O B . X . . . . . . W X B . . . . |
$$ | W W T O B . . . . O . . . X O O O . . |
$$ | T T T W W O . T T T . . O O X X X O . |
$$ | T T T T T T T T T T W W O X . X . X . |
$$ | T T T T T T T T T T W B B . X . . . . |
$$ ---------------------------------------[/go]


Black:
top left = 15
top right = 3
bottom right up to row 7 = 25 (n5 for p5 a bit kind to white, b can push out instead so might be a gote for w but if so solidifies lower side, and if b ataris n6 white gets n4 sente)
middle right around q10 = 10
top = 5 (but this group not 100% alive, for counting I assumed a-f but with support below white e could be f.
left = 4? (not alive)
Total = 59

White
right = 6 (actually 6 1/3 but nvm)
top o17 = 9
top g18 = 5
d11 area = ? if b10 is white semi-sente as b group unsettled then d13 makes ~6 points so = 3 now
lower side = ??? this is the big mystery of the game. If black takes gote to fix the ko, and given c6 isn't safe it's probably an overplay for black to 3-3, plus white has sente to fix. f5 does threaten action, so if we be generous to white and give b5 b6 exchange (not necessarily now but as consequence of c6 unsettled) and then white spends a gote at h4 then it's about 27. Also I assumed white won't m1 ko, but it's not a picnic for black either.
komi = 7.5 (chinese)
Total = 57.5.

Captures? I'm guessing white captured one at b16 and o15 and black at p2 and s18.

So by this counting white would be 1.5 behind with spending sente at h4. However, given black has 2 not quite alive groups white can likely be more active, daring black to go into the lower side leading to a potential 3-way attack. Maybe white can make a few points in the centre. So game is even within my margin of error.

About fixing the ko, although it's fairly picnic for white it's not totally free, as if black wins it white loses the g18 eyes and that big group which I had treated as settled before is no longer. So maybe black doesn't need to defend it now (and a move that helps either his weakish left or top groups and thus also weakens the white big group making the ko heavier for white would be a prime candidate) but that obviously leads to a much harder to evaluate board.


This post by Uberdude was liked by 3 people: Gomoto, Hades12, Joaz Banbeck
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 Post subject: Re: Why did he play this move?
Post #10 Posted: Tue Jul 30, 2019 4:24 pm 
Oza
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The rest of the story...

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 Post subject: Re: Why did he play this move?
Post #11 Posted: Tue Jul 30, 2019 10:55 pm 
Judan

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So comparing my count with what actually happened and the final territories:
- white immediately poked at black's top group with l16, black ignored to help left group, white captured 4 stones in sente (+8).
- black h5 jump got cut off and white made large centre territory (+14) which simultaneously fixed any invasion problems of lower side (which ended up as 22, -5 from my 27 estimate: white lost 8 in corner, gained 4 around m1). In exchange black got some of lower left corner via ko (so left group 4 -> 9 points) but h5 itself essentially became mochikomi. It did help him make a little around h10 so that group ended up as 8 not 5 points, but lost the 4 stones).
- white didn't play m11 aji keshi but o9 and then n9 gote so black got a bit less around q10 and q7 than I counted (10 -> 6).
- white captured e7 stones thanks to b9 tesuji was transfer of the d13 6 points I counted.
- black captured o6 and made a few extra points in that area (+9)
- white got n1, and in sente to boot (after he spent m3 gote).
- white got k18 connection for 4 points gote, I'd counted black k18 as probably sente at the right time on o17 group but no longer with 4 stones captured (so that has ~2 bonus value).

So in summary white got enough profit from black's 2 weakish groups.

I had a little look with LZ, it thinks black was going well (70%) before d9 peep, but that was bad because white can answer efficiently like he did in the game instead of just connecting. Peep would have been ok BEFORE b13, but after it's bad because b13 becomes very bad shape with white's b15 next (which black answered greedily, better to defend corner to avoid game sequence). In fact black making a group at c6 was bad, better to simply kosumi in centre at f8, an interesting direction of play judgement.
LZ then sees the strength of the 2 black groups as key for the next stretch of play: when white bumped at l17 it wants to tenuki to b10 (better to take care of 1 group than let white make profitable exchange in sente on the other one), and when black bump answered at j17 wants h15 to poke b shape in sente too, though c16 it thinks is even better when shown and black down to 30% (I think because h15 allows black e16 peep which makes corner sequence not work).
It doesn't like a14, wants to d10 and b10 in sente first to strengthen that group: if white doesn't c11 connect but d19 to win corner semeai then black captures in sente and then a12 is first line connection out for half the group anyway as a13 sente for corner ko and saving those stones is sente for f16 weakness.
b10 is then the key point for white instead of h18, and when white h18 black should have b10 rather than fixing the ko immediately. This is because h18 actually made white heavier at the top so he doesn't want to start and lose the ko: black would ignore n5 and m11 threats. Once black wins the ko both his weak groups are now strong so white's lower side is now full of holes which black can exploit without the yoke of 2 weakish groups around his neck.

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