This position is from a 9x9 game played some hours ago at my go club. White just ignored a black ko threat (the marked stone), so black has to prove now that the corner is dead....
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.50]TM[]
AW[cb][bc][dc][bd][ae][be][ce]AB[ca][ea][eb][ec][cd][ed][de][bf][cf][df]TR[ca]C[Black to play! Can he kill the white group?]
(;B[ba]CR[ba]
;W[dd]CR[dd]
(;B[ac]CR[ac]
(;W[ab]CR[ab]
(;B[bb]CR[bb]C[This is a false eye, so white is dead! RIGHT CHOICE])
(;B[db]CR[db]
;W[cc]CR[cc]
;B[bb]CR[bb]C[This a a false eye - white is dead! RIGHT CHOICE]))
(;W[bb]CR[bb]
(;B[ab]CR[ab]C[White is dead, W A16 isn't even sente... RIGHT CHOICE])
(;B[db]CR[db]
;W[cc]CR[cc]
;B[ab]CR[ab]C[White is dead, WA16 isn't even sente.... RIGHT CHOICE])))
(;B[db]CR[db]
;W[cc]CR[cc]
(;B[ac]CR[ac]
(;W[ab]CR[ab]
;B[bb]CR[bb]C[This is a false eye, so white is dead RIGHT CHOICE])
(;W[bb]CR[bb]
;B[ab]CR[ab]C[Now W A16 isn't even sente.... White is dead! RIGHT CHOICE]))
(;B[ab]CR[ab]
;W[ac]CR[ac]C[White lives - A18 was wrong!]))
(;B[ab]CR[ab]
;W[ac]CR[ac]C[White lives!]))
(;B[bb]CR[bb]
;W[ac]CR[ac]
(;B[db]CR[db]
;W[cc]CR[cc]
;B[dd]CR[dd]
;W[ba]CR[ba]
;B[aa]CR[aa]
;W[ab]CR[ab]C[Thanks to the liberty at A14, white is alive!])
(;B[dd]CR[dd]
;W[ba]CR[ba]
;B[aa]CR[aa]
;W[ab]CR[ab]C[White sacrifices a stone and lives]))
(;B[dd]CR[dd]
;W[ba]CR[ba]C[Too easy for white.....])
(;B[ab]CR[ab]
;W[ba]CR[ba]
(;B[ac]CR[ac]
;W[dd]CR[dd]
(;B[bb]CR[bb]
;W[ad]CR[ad]C[A nice move...! White lives.])
(;B[db]CR[db]
;W[cc]CR[cc]
;B[bb]CR[bb]
;W[ad]CR[ad]C[A nice move...! White lives.]))
(;B[bb]CR[bb]
;W[ac]CR[ac]C[White lives!]))
(;B[ac]CR[ac]
;W[ba]CR[ba]
;B[ab]CR[ab]
;W[dd]CR[dd]
;B[db]CR[db]
;W[cc]CR[cc]
;B[bb]CR[bb]C[A ko...?!]
;W[ad]CR[ad]C[No! White lives!])
(;B[db]CR[db]
;W[ba]CR[ba]
;B[ab]CR[ab]
;W[ac]CR[ac]C[White lives]))