A Curious Position
In another thread, @S_Alexander offered an interesting position that occurred in one of their recent games.
Spoiler: strategically speaking, this local position most likely would ultimately be a seki, unless it turns out to be useful to sacrifice as a potential ko threat. However, even without an ongoing ko fight, understanding why this is a seki involves some interesting considerations in regards to the rules. Clearly, any further move by Black would be self-atari, although which self-atari crucially matters for finding an effective ko threat. White, on the other hand, could also self-atari to make a ko threat, but also have potential moves that would lead to some interesting considerations of the rules.
Possible Usage as a Ko Threat
Let’s first consider, hypothetically, if there was a large, ongoing ko fight elsewhere on the board.
By Black
If White has just taken the ko, and it’s Black’s turn to find a threat, then Black could potentially consider playing T3 as a ko threat. Of course, this would be self-atari, which would only strategically make sense if potentially winning the ko fight was worth offering the sacrifice of this large corner. If the ko fight elsewhere was indeed worth a very large amount of points, then White might even ignore this threat (to end the ko elsewhere), which would allow Black to then play T1, which would convert the status of this corner from a seki into another ko that would need to be settled.
Note that if Black instead attempted to play R2 as a ko threat, that would be a huge blunder that should be ignored by White, allowing White to win the ko fight elsewhere. Then, even if Black later followed up on the threat by playing at T1, White could just take back with S1, which would turn Black’s group into a dead “Bent Four in the Corner”. Hence, Black playing at R2 would only die in gote, since R2 by Black does not require an immediate answer by White.
By White
If Black has just taken a ko elsewhere, White might consider to play in this corner as a ko threat. White could do so by playing at T3, which would prompt a response by Black at T1, if responding to this threat is worth more than winning the ko elsewhere. White would now no longer have any immediate ko threats in this unsettled corner position. White could eventually take back at T2 to reset the local position into a seki, or Black might eventually follow-up with R2 to eliminate white from this corner (to gain some territory and captures), however, both of those moves are gote, which makes them ineffective as ko threats.
On the other hand, White could instead play a ko threat with R2, which would necessitate a response from Black at T1 (assuming that this threat is big enough). The seki position has now collapsed and White’s remaining stone in the corner is dead. However, White could eventually play at S1, perhaps as another ko threat, which would require Black to respond at R1 in order to prevent White from coming back alive (in seki) with a followup play at R1 to nullify Black’s territory in the corner.
Note that after the sequence W R2, B T1, W S1, B R1, White’s stones are dead, and Black is fully alive (not in seki) in that corner. This position is settled, and neither player should waste any more moves in that corner. However, it is of interest to note that this is an example of a “Dead Ko”, where playing on to actually capture the White stones could start a ko pattern, but White can never “win” this ko by resolving it in their favor. Ultimately, this position should be ruled dead during the confirmation phase, however, this situation becomes very peculiar when elsewhere on the board there is also a source of infinite ko threats (such as the double ko seki that we discussed earlier), which would cause this to become an example of Moonshine Life. I will discuss Moonshine Life in more detail in a future post.
A Cycle that Does Not End with “No Result”
Now let’s consider if there are no ko fights elsewhere and everything else on the board is fully settled, and imagine that White is behind in points (with this corner shape called as a seki).
In this case, although the position should locally be a seki, it is possible for White to still force the game to continue by playing the 3-move “sending two, returning one” cycle:
- White at T3
- Black at T1
- White at T2
- Black passes
Note that White yields two prisoners to Black in each cycle, while only getting one back, hence the name “sending two, returning one”.
Assuming that the game is close enough that Black cannot afford to lose those stones, this cycle is forced to continue (at least for a while), and thus White could force the whole-board position to repeat. White might mistakenly think that they could claim that this game is now a “no result” (which they might prefer over conceding the loss), however, recall the exact wording of article 12:
Article 12. No result
When the same whole-board position is repeated during a game, if the players agree, the game ends without result.
The crucial words are “if the players agree”. If White tried to pull such shenanigans of forcing a “sending two, returning one” cycle in order to attempt to claim a “no result” for cycling, Black should simply refuse. The Japanese rules do not have a superko rule, so this cycle can continue, if White persists, however, since Black gets a net gain of one prisoner in each cycle, eventually Black will gain enough points to simply abandon their stones in the corner and still win. Thus, there’s no strategic advantage for White in persisting with this cycle, as all they would accomplish is to lengthen a lost game, which would probably be viewed as rude (if White understood what they were doing).