Sending two, returning one
Sending two, returning one is a special case of capture two, recapture one where one player sends two stones to be captured by putting them into self-atari before recapturing one stone, as the following diagrams illustrate.
This corner is seki. However,....
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Black can play -
. Such play seems ridiculous, but it costs nothing by area scoring. (Black loses a point by territory scoring.) If the rules allowed Black to do so, he could extend the game indefinitely, or both players could collude to do so. Different rules deal with sending two, returning one differently.
An aspect of territory scoring is that one player may have a related way to prolong the game while losing points. (See example on the Tsumego From Games 21 page.)
Under area scoring a play inside territory to prevent this would cost nothing, right at the end of the game. That isn't the case under territory scoring; it may be practical to give back one point eventually.
Actually it's area scoring that has a problem with "sending two, returning one".
Without some super ko rule, this 4x3 game wouldn't end:
- Black will endlessly attack instead to lose by 6 - 4 = 2 points.
Under territory scoring, on the other hand, White (leading by, say, 1 prisoner) would simply give up the whole board at the moment she piled up enough black prisoners - 26, I guess. :-)
Harry Wang: Actually, in GO, endless attack by black is NOT allowed. The game ends with white winning over black.
In case suicide is an option and Black commits it (clearing the whole board), the game will continue as shown, forcing Black to pass with . White will pass too - if she doesn't want to make fun and start "sending two, returning one" herself. ;-)
- Bill: Example, please. I do not believe I have ever seen a sending-2-returning-1 position that could be prevented by a play inside one's territory, except by preventing the seki in the first place, which would be a sizable play under either scoring style.
Charles I was thinking of something like this.
White at a is a threat (to make seki) so that Black must capture two; White then can recapture one to repeat the board position. If Black eventually plays b that prevents the repetition.
- Charles Matthews Ah yes - with another external liberty for Black it would threaten seki.
- Bill: Thanks, Charles. Interesting position. But the threat to prolong the game is slight.
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Now Black can play at with no net loss, regardless of scoring method.
There may be no such play, as in the case just cited and also in the example on the superko page. Then in principle under territory scoring one allows the opponent to do it 100 times (say) and then passes instead of continuing round the cycle, allowing the opponent to take a few stones.
Under a superko rule such positions cannot be repeated indefinitely.
Had something like this in my game and found an interesting solution. We played under japanese rules. W sends 2 and returns 1, B passes, W again sends 2 with returning 1,B passes, W makes it again. If b would pass game will be end as a draw (played on KGS - dont sure about other systems) - so b fills 1 point inside his territory otherwhere (loses 1 point according to japanese rules). W repeats process. So at each cycle B gains 2 ponts. Even if that ko worth 100 pts b can give it up after 51 cycle^). So main problem - if B has enough inner territory to do this.
PS. Under Chinese rules B takes 3 point at each cycle - or i m wrong? Aret
See also: