(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.00]
GN[Black to play]DT[2004-03-26]PC[http://senseis.xmp.net/?UnderTheStonesProblem6]AW[bb][eb][cc][dc][cd][ed][be][ce][de][ee]AB[cb][db][fb][bc][ec][fc][bd][fd][ge][ff][bg][cg][eg]C[Black to do his best.]PL[B]
(;B[ab]CR[ab]
;W[ba]CR[ba]
(;B[ac]CR[ac]
(;W[ad]CR[ad]
;B[ae]CR[ae]
;W[af]CR[af]
(;B[bf]CR[bf]
;W[ad]CR[ad]C[RIGHT Ko is best.])
(;B[ag]CR[ag]
;W[ad]CR[ad]C[RIGHT Ko is best.])
(;B[ea]CR[ea]
;W[ad]CR[ad]LB[id:T]C[If you think this is a ko, play a ko threat at T.]
(;B[id]CR[id]C[RIGHT Correct. Black will play C19 to win the ko if he chooses to ignore a ko threat.])
(;B[ca]CR[ca]
;W[aa]CR[aa]
;B[bc]CR[bc]
;W[bf]CR[bf]C[White lives.])))
(;W[ae]CR[ae]
;B[ea]CR[ea]
;W[ad]CR[ad]
;B[ca]CR[ca]
;W[aa]CR[aa]
;B[bc]CR[bc]C[White dies unconditionally. White A15 was a mistake.]))
(;B[ea]CR[ea]
;W[ad]CR[ad]
(;B[ca]CR[ca]
;W[ac]CR[ac])
(;B[ac]CR[ac]
;W[aa]CR[aa])))
(;B[ba]CR[ba]
;W[ab]CR[ab]TR[bc][bd]C[White lives. Marked stones are dead.]))