(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[]PB[Black to Play]C[Just play through the moves to move 20 which is the problem
position. These do not represent a game sequence!]
;B[pq]
;W[pp]
;B[op]
;W[po]
;B[qp]
;W[qq]
;B[pr]
;W[qr]
;B[qs]
;W[rs]
;B[ps]
;W[qn]
;B[rr]
;W[rq]
;B[rp]
;W[sq]
;B[ro]
;W[rn]
;B[nq]
(;W[]C[This is the problem position. Black to play
and capture some white stones and rescue three
of his. 

This is the correct diagram position. The published
version in BGJ140 had an extra white stone at T4.]
(;B[sr]C[CORRECT: This move just looks like it 
sacrifices an extra black stone, but it is the key move for
reducing white's liberties here.]
;W[ss]
;B[sr]
(;W[so]
;B[rr]C[White can't connect at T4 without remaining in atari.

S1 is not quick enough as he just gets captured at T4.])
(;W[rr]
;B[sp]C[White is now out of liberties.])
(;W[sp]
;B[so]
;W[rr]C[This fails as well.]))
(;B[sp]
;W[sr]C[FAILURE: White captures one stone to give two eyes.
Remember two eyed groups never die!])
(;B[ss]
;W[so]C[FAILURE: It is self-atari for black to play either T4 
ot T2 so white escapes capturing three stones,
and black has gained little in the corner.]))
(;W[sp]C[This is the incorrect position as published in BGJ 141.
We apologise for this error that somehow didn't get
corrected after proof reading (an earlier error without
the stone at T3 was spotted). 

Anway there is still the interesting question of what
the correct black move is and how many ko-threats
the position has for black.

]
(;B[ss]
;W[so]
;B[sn]
(;W[qo]C[Taking the stone on S2 is worth some points,
but there are only two ko-threats here.

White now gets a small ko threat against the 
two corner stones.])
(;W[sm]C[This is a mistake as black gets another 
ko threat.]
;B[sr]
;W[rs]C[Though the extra black ko threat is suicidal.]))
(;B[so]
;W[sr]C[This is failure to capture and only one ko-threat.])
(;B[sr]C[CORRECT: To play the same move as in the correct
problem.]
;W[ss]
(;B[rr]
;W[so]
;B[sr]
;W[qo]
(;B[rs]C[Playing the same move as the correct problem solution
gives 3 ko threats and the possible follow up of S1
to take some points in the corner in gote.])
(;B[]
;W[rs]C[This is also gote for white but the difference is 6 points 
(the two black stones are captured in a snapback shape).]))
(;B[so]
;W[sn]
(;B[sr]
;W[qo]C[This line gives 3 ko threats as well, but this loses
points as the stone on T2 is captured (it's a snapback
shape if black tries to capture on S2).])
(;B[rr]
;W[qo]C[This ends the same as the previous variation.])))))
