(;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.]))