Re: [gnugo-devel] auto generated regression tests

[Gunnar Farneback]

Dan wrote:
> Sorry, A19 is obviously wrong and must be a typo. What I think I intended was
> A18. But I was wrong that it is inevitably seki.

Ok.

> B19 and A17 produce kos but not as good. For example
> 
> B:B19 W:A18 B:A17 and W gets the first ko capture. 
> 
> On the other hand if:
> 
> B:A18 W:A17 B:B19 W:A19
> 
> this is mannenko or 10,000 year ko. W can make seki at any time
> by connecting A18. B has no way to make seki. Either player can
> start an unfavorable ko. Whichever player starts the ko has to
> make the first ko threat.
> 
> See: http://senseis.xmp.net/?ThousandYearKo
> http://www.msoworld.com/mindzine/news/orient/go/history/oteai2.html
> 
> In order to start a ko, W must first fill the
> outside liberties at F16, E12 and E14, then make atari at C17
> or A16. But B gets the first ko capture.
> 
> On the other hand if B wants to start the ko he has to take
> the ko, then fill a liberty at C17 or A16. If he does not
> take the ko before filling a liberty, W connects and B
> has a dead eye shape. Thus W gets the first ko capture.

There must be something missing in this analysis. If black starts with
A17 we get this position after four moves:

   A B C D E F G H
19 . X X X O O O X
18 O O X X X O X X
17 . O . X O O O X
16 O X X X O . O O
15 X X O X X X X O
14 X O O X . X X O
13 O O X X O X O O
12 . O O O . O X O
11 . . . O . O X O

and if black starts with A18 this position:

   A B C D E F G H
19 O X X X O O O X
18 . O X X X O X X
17 O O . X O O O X
16 . X X X O . O O
15 X X O X X X X O
14 X O O X . X X O
13 O O X X O X O O
12 . O O O . O X O
11 . . . O . O X O

(If black starts with B19 white gets the choice of ko, which does not
improve anything for black.)

Similarities between the two positions:
* White has captured the ko in the previous move.
* White can make seki by filling the ko.
* If white wants to capture black he must first fill the outer
  liberties (or at least one of them) followed by one common liberty.
  Then black takes the ko and white must find the first ko threat.
* Similarly if black wants to live with territory he must first take
  the ko, then fill a common liberty and win the ko once more.

Differences between the two positions:
* In the lower diagram black has no way to force seki. In the upper
  diagram black can immediately play the ko and fill it to get seki.

Or put in other words both positions are mannenko with the difference
that in the upper diagram both players can make seki but in the lower
diagram only white.

To me it looks like the upper diagram is clearly better for black and
hence that A17 is the correct move.

Am I missing something?

/Gunnar