There is an interesting end of game protective play question that arose. It would not make any difference to winning or losing, but it is something to be aware of. Even fairly good SDKs miss such plays from time to time.
- Click Here To Show Diagram Code
[go]$$Bc Black to play (move 207)
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . X . . . . . . . . . . . |
$$ | X X X X X W X . . . . . . . X . X X X |
$$ | W X W W W W X . . , . . . . . , X O O |
$$ | W W . c . W W X X . . . . . . X O . O |
$$ | . . a W X W X O O X X X . . . X O O . |
$$ | . . b W W X X O O O O X . . X O . . . |
$$ | . . X W X O X O . . O X X . X O . . . |
$$ | . . O X X . X O X O O O X X O O . . . |
$$ | . . O O X . X X O , O X X O X , X . . |
$$ | . . O X X X O X O O O . X O O O . . . |
$$ | . . O X O O O O X X O X X X X O O . . |
$$ | . O X X X X O X X . X X O O O . . O O |
$$ | . O X . X O O . X . X X X O . . . O X |
$$ | . . O X X X O O O X X . O O . O O X X |
$$ | . . O O X X O . X , X . O X O O X X . |
$$ | . . . O O O O X X . X X O X X X X . . |
$$ | . . . . . . . O O X O O X X . . . . . |
$$ | . . . . . . O O X X . . . . . . . . . |
$$ ---------------------------------------[/go]
The three marked Black strings have 3 dame each and share a total of 7 dame. As Wilcox warns us, they may be in danger. Black considered a play at "a", "b", or "c", did not find that any of them worked, and passed. Afterwards, Leela pointed out the cut.
There is a proverb that would have helped Black and saved him some time. It applied to life and death, of which this is an example, When you are considering how to kill, it says, "First, the hane, then the cut, then the oki." You might consider "c" a hane, but the proverb refers to hane from the outside. "a", "b", and "c" are not oki, but the primacy of the cut still applies. If Black had started with the cut, he might well have discovered that it works.
What is so good about the cut in this position (and in general)? It reduces White's dame. OC, so do the other plays, but the cut "keeps on giving".
- Click Here To Show Diagram Code
[go]$$Bc Black wins the semeai
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . X . . . . . . . . . . . |
$$ | X X X X X W X . . . . . . . X . X X X |
$$ | W X W W W W X . . , . . . . . , X O O |
$$ | W W 1 4 . W W X X . . . . . . X O . O |
$$ | 5 3 2 W X W X O O X X X . . . X O O . |
$$ | . a 7 W W X X O O O O X . . X O . . . |
$$ | . . X W X O X O . . O X X . X O . . . |
$$ | . . O X X . X O X O O O X X O O . . . |
$$ | . . O O X . X X O , O X X O X , X . . |
$$ | . . O X X X O X O O O . X O O O . . . |
$$ | . . O X O O O O X X O X X X X O O . . |
$$ | . O X X X X O X X . X X O O O . . O O |
$$ | . O X . X O O . X . X X X O . . . O X |
$$ | . . O X X X O O O X X . O O . O O X X |
$$ | . . O O X X O . X , X . O X O O X X . |
$$ | . . . O O O O X X . X X O X X X X . . |
$$ | . . . . . . . O O X O O X X . . . . . |
$$ | . . . . . . O O X X . . . . . . . . . |
$$ ---------------------------------------[/go]

@ 1
Leela's main line shows White's most stubborn resistance. Here is White's combined dame count, move by move.

6 dame

5 dame

4 dame

4 dame

3 dame

2 dame

1 dame

makes

and

atari. Note that

and

each reduce the White dame count. This is the kind of thing that makes such cuts work.

Note also that White should play at "a" instead of

, and give up the three stones.
- Click Here To Show Diagram Code
[go]$$Bc Variation
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . X . . . . . . . . . . . |
$$ | X X X X X W X . . . . . . . X . X X X |
$$ | W X W W W W X . . , . . . . . , X O O |
$$ | W W 1 2 7 W W X X . . . . . . X O . O |
$$ | . 4 3 W X W X O O X X X . . . X O O . |
$$ | . . 5 W W X X O O O O X . . X O . . . |
$$ | . 6 X W X O X O . . O X X . X O . . . |
$$ | . . O X X . X O X O O O X X O O . . . |
$$ | . . O O X . X X O , O X X O X , X . . |
$$ | . . O X X X O X O O O . X O O O . . . |
$$ | . . O X O O O O X X O X X X X O O . . |
$$ | . O X X X X O X X . X X O O O . . O O |
$$ | . O X . X O O . X . X X X O . . . O X |
$$ | . . O X X X O O O X X . O O . O O X X |
$$ | . . O O X X O . X , X . O X O O X X . |
$$ | . . . O O O O X X . X X O X X X X . . |
$$ | . . . . . . . O O X O O X X . . . . . |
$$ | . . . . . . O O X X . . . . . . . . . |
$$ ---------------------------------------[/go]

and

save the three stones, but then

is atari on the other

stones. OC,

at 5 loses only the three stones, but playing

in the previous diagram is the better way to do that.
Edit: For completeness.
- Click Here To Show Diagram Code
[go]$$Bc Snapback
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . X . X . . . . . . . . . . . |
$$ | X X X X X W X . . . . . . . X . X X X |
$$ | W X W W W W X . . , . . . . . , X O O |
$$ | W W 1 3 . W W X X . . . . . . X O . O |
$$ | . 2 a W X W X O O X X X . . . X O O . |
$$ | . . b W W X X O O O O X . . X O . . . |
$$ | . . X W X O X O . . O X X . X O . . . |
$$ | . . O X X . X O X O O O X X O O . . . |
$$ | . . O O X . X X O , O X X O X , X . . |
$$ | . . O X X X O X O O O . X O O O . . . |
$$ | . . O X O O O O X X O X X X X O O . . |
$$ | . O X X X X O X X . X X O O O . . O O |
$$ | . O X . X O O . X . X X X O . . . O X |
$$ | . . O X X X O O O X X . O O . O O X X |
$$ | . . O O X X O . X , X . O X O O X X . |
$$ | . . . O O O O X X . X X O X X X X . . |
$$ | . . . . . . . O O X O O X X . . . . . |
$$ | . . . . . . O O X X . . . . . . . . . |
$$ ---------------------------------------[/go]
If

is at "a",

at "b" captures the two Black stones.