This problem demonstrates another simple but useful endgame technique. White just played the marked stone - how can Black win after this?
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[7]KM[0.50]
AW[da][cb][cc][dd][be][ce][de][af][cf]AB[db][dc][ed][ee][bf][df][ef][bg][dg]TR[da]C[No komi, no prisoners - Black to play and win!]
(;B[fb]CR[fb]
(;W[ea]CR[ea]
;B[cg]CR[cg]
;W[eb]CR[eb]
;B[ec]CR[ec]
;W[fa]CR[fa]
;B[gb]CR[gb]C[Black wins by one point! RIGHT CHOICE])
(;W[cg]CR[cg]
;B[ea]CR[ea]
;W[ca]CR[ca]C[Black wins by one point! RIGHT CHOICE])
(;W[fa]CR[fa]
;B[eb]CR[eb]
;W[ea]CR[ea]
;B[cg]CR[cg]
(;W[gb]CR[gb]
;B[gc]CR[gc]
;W[fc]CR[fc]
;B[ga]CR[ga]C[White can't play this ko])
(;W[ca]CR[ca]
;B[gb]CR[gb]C[Black wins by three points!])))
(;B[ea]CR[ea]
;W[eb]CR[eb]
;B[ca]CR[ca]
;W[ec]CR[ec]C[Black is lost])
(;B[eb]CR[eb]
;W[ea]CR[ea]
;B[fa]CR[fa]
;W[ca]CR[ca]
;B[fb]CR[fb]
;W[cg]CR[cg]C[White wins by one point!])
(;B[cg]CR[cg]
;W[eb]CR[eb]
(;B[ec]CR[ec]
;W[fb]CR[fb]C[This is an easy win for white])
(;B[fb]CR[fb]
;W[ec]CR[ec]
;B[fc]CR[fc]C[White wins!])))
