Hi!

I have a question about a particular tsumego exercise at GoChild
Problem 2763 (Dong-Dong grouping):

https://i.imgur.com/G3SpYF0.png

http://eidogo.com/#4tsEbyN8q

It gives two correct answers for black. Playing either T19 or T18.
The way I understand the problem, playing T19 kills the corner ("bent four in the corner").

https://senseis.xmp.net/?BentFourInTheCorner

Playing T18 starts a ko and if white wins the ko, white can live.

So basically I would expect T18 to be wrong, because if you can just kill the corner,
why would you want to give white the opportunity to live by winning the ko?

Or does it somehow depend on the rule-set that is being played under, which might determine
whether or not "bent four in the corner" has to be played out?

greetings and thanks in advance for any feedback on this issue.

Even if bent four has to be played out, T19 is better than T18.

It's weird for sure. Maybe the idea behind T18 being accepted as a valid answer is that Black will not play either of both moves until the very end of the game. He can then be assured that white does not have anay ko threat left. If he, Black, still has some, then T18 does the job.

That being said I agree that T19 is simpler and safer, whichever ruleset is in use.

For reference, the position is:

Click Here To Show Diagram Code

[go]$$B Black to play: a or b?
$$-----------------+
$$ . . X O . X X a |
$$ . . X O O O O b |
$$ . . X X X X O O |
$$ . . . . . . X X |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

(See How to make diagrams. If you copy-paste from examples on that page, it's pretty quick and easy.)

The choice comes down to whether black is forced to remove the white stones during the game, or whether the black stones on the outside are independently alive.

This is a bit far-fetched, but the best I could come up with: suppose there is a ko fight elsewhere on the board, white plays as a ko threat and black ignores it:

Click Here To Show Diagram Code

[go]$$W B2 resolves ko elsewhere
$$-----------------+
$$ . . X O . X X a |
$$ . . X O O O O b |
$$ . . X X X X O O |
$$ . . . . . 3 B B |
$$ . . . . . 1 . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

Now black a is too slow. If black wants to save the marked stones and capture the corner, then b is the only way.

Alternatively, later in the endgame it might look like this:

Click Here To Show Diagram Code

[go]$$B Black to play: a or b?
$$-----------------+
$$ O . X O . X X a |
$$ O . X O O O O b |
$$ O . X X X X O O |
$$ O . . . . . X X |
$$ . O . . . . . . |
$$ . . O O O O O O |
$$ . . . . . . . . |[/go]

Yeah, I guess one can kind of imagine a context that justifies each of the given
answers. But often the logic of the tsumego seems to be that you have to consider only
the pattern given and then certain responses are not accepted because they are
not optimal (like leaving more ko-threats while otherwise obtaining an equivalent
result).


A closely related problem at GoChild is Problem 4631 (Dong-Dong grouping):


https://i.imgur.com/RgkRHRB.png

Click Here To Show Diagram Code

[go]$$B Black to play.
$$-----------------+
$$ . . . O . X X . |
$$ . X . O O O O . |
$$ . . X . X O O O |
$$ . X . . X X X X |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

In this case, where white has outside liberties, only the move in the
corner at L11 is accepted as a solution. This makes sense, because
white would immediately be alive by responding at H11 if black plays at L10.

If you put letters on the diagrams here, it's much easier to read...

Click Here To Show Diagram Code

[go]$$B Black to play.
$$-----------------+
$$ . . . O c X X a |
$$ . X . O O O O b |
$$ . . X . X O O O |
$$ . X . . X X X X |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

Black a is the only correct answer here. If black b, then white c. If you've got two or more outside liberties, then you can squeeze.

Yeah, but the downside is that it kind of gives away the answer in the diagram, so if possible
I'd prefer to have custom coordinates on the diagram.
I didn't see how to do that, so I just referred to the coordinates visible in the imgur
screenshot.

Where your diagram code has "$$B Black to play" at the top, just put in a 'c' for coordinates: "$$Bc Black to play", like this:

Click Here To Show Diagram Code

[go]$$Bc Black to play.
$$-----------------+
$$ . . . O . X X . |
$$ . X . O O O O . |
$$ . . X . X O O O |
$$ . X . . X X X X |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

It's all there in the instructions at How to make diagrams.

To get coordinates matching your screen shot, you need to tell it you're on an 11x11 board. So that's "$$Bc11":

Click Here To Show Diagram Code

[go]$$Bc11 Black to play.
$$-----------------+
$$ . . . O . X X . |
$$ . X . O O O O . |
$$ . . X . X O O O |
$$ . X . . X X X X |
$$ . . . . . . . . |
$$ . . . . . . . . |
$$ . . . . . . . . |[/go]

You can also put and around a block of text if you want to hide an answer,



Diagrams are the best feature of the site (next to embedded SGFs). We've got features that make it easier to discuss go positions, no need to open screen shots in another window to see what's going on. That's why I'm here, not on reddit or facebook.