粘劫收後, as in 粘 zhān stick 劫 jié ko 收 shōu collect 後 hòu behind, is a fake chengyu. Completely unpoetic and with no use outside of Go discussions, it simply means “connect the ko and get the last dame”.
This happens regularly in Chinese rules. (I guess that it plays a role under any rule set that cares about the dame points.)
The conditions for this to happen (and matter) are as follows:
- It is Black’s turn and the last obvious move (besides the dame) is to connect a so-called half-point ko.
- However, if Black does so, the result will be B 184=W+0,5.
- The amount of dame is an odd number.
- Black has an abundance of ko threats.
- Thus, Black can play the dame while holding onto the ko and get the last dame.
- After getting the last dame, White has no ko threats and so is forced to pass.
- Black connects the ko and wins with B 185=B+1,5.
(Of course, you can interchange “Black” with “White” and switch the numbers.)
Note that the amount of dame must be odd. Otherwise winning the ko would be no different from playing two dame elsewhere.
This takes us back to the game I mentioned previously:
We know that the result was B 184=W+0,5.
The losing move was this seemingly innocuous connection of the last ko, move 231.
We can see that there is an odd number of dame, fulfilling the first condition for 粘劫收後. Therefore, the game will become a battle of ko threats if Black plays a dame instead.
I find that 1 would be an excellent dame for Black to play.
It turns the ko to a pure “half-point ko” (so that White taking it is not atari on another stone) and also prepares ko threats at A.
As good practice dictates, White removes the double-threat of 2-3 before taking the ko.
Let’s say Black makes a normal ko threat and retakes the ko.
Now White has two ways to go about the ko:
1) Play normal threats and try to win the ko.
(That would be an instant win.)
I marked White’s threats with numbers (12 connects at 9), and Black’s threats with letters. (If White answers I at L, Black can still use M and N as threats, so it’s the same.)
R is the 18th letter in our alphabet, which means Black has three more threats.
However, White has another way:
2) White can play dame as threats.
Black would need to respond by playing another dame, in order to achieve 粘劫收後. (Otherwise White still wins.) The decision of the dame point is again related to ko threats.
A gives two more atari-threats, B decreases Black’s threats by one, C gives a throw-in after cutting. But whatever White does, Black will take the dame at D.
This dame 9 gives Black at least one more ko threat.
(If I didn’t make a mistake, it turns out that cutting at C and D is the same in terms of threats. It’s really tricky though…)
Now we also notice that the point I marked with 11 might be another double-threat. Should White have played there before 4? Because Black could have threatened there instead of 7?
If I were to deliver a final verdict on who would have won with the optimal play, it would probably not be entirely correct. (Some people told me that I would have lost, and others said the opposite.)
You can see that it is extremely unclear whether Black could have pulled of 粘劫收後. But whatever would have happened, he should have tried. It is likely that, due to byo-yomi, both sides would have made several errors playing and responding to threats and choosing the best dame points, so the outcome could as well have been decided by a coin flip.
It pays to know the rules~