Life In 19x19

https://lifein19x19.com/download/file.php?id=9152

(;ST[2]AP[GOWrite:2.3.46]GM[1]CA[ISO8859-1]SZ[19]FF[4]PB[ ]C[BLack to play wins.

White to play normally makes ko.

Initial plays gain around 8 pts., depending on the ko threat situation.]AW[db][eb][fb][cc][bc][bd][ce][cg][ci]FG[259:]PM[2]PW[ ]GN[ ]AB[bb][cb][ac][dc][ec][fc][gc][gb][jc][dd][fe]
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;C[Black to play wins.]B[ab]
;C[*** A 1 pt. sente.]W[ad]
;B[fa]
)
(
;C[*** This tesuji is White's normal play.

First, let's look at the play when White is komaster.

White should normally play this move when Black is komaster, and take pts. elsewhere in exchange for the ko, but see variation for a different move.]W[ba]
(
;C[*** Black's normal play, except when Black is komaster. See variation.]B[ca]
;C[*** After this play, when White is komaster, plays gain a little more than 5 pts. When neither player is komaster, plays gains somewhat less. This is a normal place for Black to play elsewhere. See continuations.]W[ad]
(
;C[*** We are assuming that Black is not komaster here. In that case this is her best continuation. With this play Black limits her losses if she must lose the ko. See variation for Black's play as komaster.]B[fa]
;C[*** Black gets to take the ko first.]W[ab]
)
(
;C[*** It is possible for Black not to be the komaster for the larger ko, but to be komaster for the smaller ko. Black lets White take the ko first, but has the possibility of winning the whole thing.]B[aa]
)

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;W[ab]
(
;C[*** Black takes the ko first, but it is an uphill battle. See variation.]B[aa]
)
(
;C[*** The position when White wins the ko, as expected. See continuation.]W[da]
;C[*** White has a 3 pt. sente. Which is one reason why, when Black is koloser, she wants to play F-19 and let White make the ko.]W[ga]
;B[ha]
;W[fa]
;B[hb]
)

)

)
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;C[*** Black's play as komaster.]B[da]
(
;C[*** Throw-in ko. White should usually play this way.

See variation.]W[ab]
;C[*** Black takes first. As koloser, White should often play elsewhere at this point. But see continuation.]B[aa]
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;W[ad]
;C[*** Forced. Plays in the remaining ko gain 4 1/3 pts.]B[ca]
)
(
;C[*** Position if Black wins the ko, as expected.]B[ca]
;C[*** A 1 pt. sente.]W[ad]
;B[ab]
)

)
(
;C[*** Usually better, as koloser, to make the throw-in ko and gain pts. elsewhere while Black wins the ko. But sometimes White plays here.]W[ad]
;B[fa]
;W[ab]
;C[*** White loses the semeai, but makes something in the corner.]B[ea]
)

)

)

(
;C[*** This play is a losing sente, and will not usually be a realistic option. It loses less than 3 pts. ]W[ad]
;C[*** But it gives Black this nice tesuji. Wonderful things happen on the 1-2 point!]B[ba]
;W[ab]
;C[*** White loses the semeai, but gains a bit in the corner with sente. ]B[fa]
)

)