Had this in one of my recent games and since we were using jap rules we couldn't play it out in the game:

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[go]$$ Status?
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . O . X O . .
$$ - . O . X O . .
$$ - . . X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

_________________
Sanity is for the weak.

Looks like seki to me.

Depends who goes first.

If Black goes first, then it's either a seki or a ko which is White's privilege:

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[go]$$B Ko
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 2 O . X O . .
$$ - 3 O . X O . .
$$ - . 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Click Here To Show Diagram Code

[go]$$B Seki
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . O . X O . .
$$ - 2 O . X O . .
$$ - . 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

If White goes first, then it's dead:

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[go]$$W Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . O . X O . .
$$ - . O . X O . .
$$ - . 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

This move makes it look like it's seki but it is not the case. If Black fills at either 'a' or 'b', then it dies. However, White can play at either 'a' or 'b' to threaten an eye inside, but Black can't save it or else it dies in a liberty shortage. I don't think is even necessary, but it helps you see the shape better.

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[go]$$W edit - (NOT) Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - a O c X O . .
$$ - 2 O 3 X O . .
$$ - b 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

edit: It's not dead! Black can play at 'c' and it becomes seki.

Black can't play at 'a' due to liberty shortage, but a White stone there only makes the dead shape (bulky 5) inside. (again, is not necessary here either, but just to make it easier to see)

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[go]$$W Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 4 O . X O . .
$$ - a O 2 X O . .
$$ - 3 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

If Black doesn't play inside and refuses to believe it is dead, you can show this shape (rabbity 6):

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[go]$$W Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 5 O . X O . .
$$ - 7 O 3 X O . .
$$ - 6 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

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[go]$$W Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . . 8 X O . .
$$ - . 9 . X O . .
$$ - X . X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

What if black 4 plays on the unmarked point?

rofl, wow...I'm an idiot. Yeah I thought if Black goes there, the entire group self-atari's. Sigh...let me go re-evaluate this.

I think the result is seki, and the best move for both sides is to pass. The most interesting case is if W tries to kill, but it looks like B can defend successfully:

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[go]$$W Seki
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . O 4 X O . .
$$ - 2 O 3 X O . .
$$ - . 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Click Here To Show Diagram Code

[go]$$W Seki
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 3 O . X O . .
$$ - 2 O . X O . .
$$ - 4 1 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Okay so what I said about Black going first doesn't change, but if White goes first, I don't think she can unconditionally kill. Rather, it becomes ko or seki depending on whether White chooeses to play 'a' or 'b':

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[go]$$W Ko or seki, White's privilege
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 1 O . X O . .
$$ - b O . X O . .
$$ - a 2 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

doesn't work because after , White can make the rabbity six or a bulky 5 if Black plays another move inside.

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[go]$$W Dead
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 1 O . X O . .
$$ - 3 O . X O . .
$$ - 2 . X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Not the same as this:

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[go]$$W Seki
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - O O . X O . .
$$ - O O . X O . .
$$ - . X X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Ah, looks like we finally have an answer? B has nothing constructive to do, so he should pass. W should then play below. B must answer (since a W move at that point kills unconditionally). W can then bide his time and look for an opportunity to play "a" to prepare a ko. B should still pass, since W only gets seki by filling the ko. If W spends another move to take a shared inside liberty, B gets to capture the ko first. This is a mannen-ko, pretty difficult for W to win.

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[go]$$W Ko
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 1 O . X O . .
$$ - . O . X O . .
$$ - a 2 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

Is it so difficult? Since White can always play , and it's just 1 stage.

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[go]$$W Ko
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - O O 1 X O . .
$$ - . O . X O . .
$$ - 3 X X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

But white needs an approach move before starting the ko, and then black takes first. On the other hand, black can start the ko at any time convenient for him, and then it's an approach-ko, that is, white needs to win it twice.

What about that:

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[go]$$W simple ko, white takes first
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - 3 O . X O . .
$$ - 2 O 1 X O . .
$$ - 5 4 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

White also has the option of sente-seki by playing at .

It is certainly a risk for B, but how large?

It takes W three moves to complete a 25 point capture. In that time B gets a move somewhere else, plus a ko threat and execution. Early in the endgame, when there are plenty of large gote moves, that may well be enough compensation for B.

Late in the endgame, B may be able to fight the ko, since ko threats are likely to be small. If W starts the ko and loses, it costs 10 points (compared to seki), so B can ignore a ko threat that large. Of course B has to come up with larger ko threats to continue fighting the ko.

At the very end of the game, I guess it might depend on whether there are removable ko threats or not. It also depends on the rule set, whether it costs B points to remove ko threats within his own territory.

Mannen-ko is not simple

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[go]$$W gote seki
$$ - . . . . . . .
$$ - O O O O O . .
$$ - X X X X O . .
$$ - . O . X O . .
$$ - 3 O 1 X O . .
$$ - . 2 X X O . .
$$ - X X X O O . .
$$ - O O O O O . .
$$ - . . . . . . .[/go]

If one space up , then it becomes the same situation as my previous post, just the stone filling a different space.

The solution according to gotools is Ko with White needing one external ko threat to kill.

_________________
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Go is such a beautiful game.