This is the seventh and last 7x7-endgame-ko-problem! The last white move (the marked stone)was a mistake, although a common one (A2 instead would have secured a jigo). How can black punish white for his careless play?
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[7]KM[0.50]TM[]
AW[ec][cd][ed][fd][gd][ae][ce][de][ee][bf][ff]AB[fa][db][eb][bc][cc][dc][fc][gc][bd][dd][be]TR[ae]C[No komi, no prisoners - black to play and win!]
(;B[af]CR[af]
;W[ag]CR[ag]
(;B[cf]CR[cf]
;W[df]CR[df]
(;B[cg]CR[cg]
;W[dg]CR[dg]
;B[ad]CR[ad]
;W[bg]CR[bg]
;B[af]CR[af]
;W[fb]CR[fb]
;B[gb]CR[gb]
;W[ae]CR[ae]
(;B[cf]CR[cf]
;W[cg]CR[cg]
;B[af]CR[af]C[Black connects the ko and wins by one point! RIGHT])
(;B[cg]CR[cg]
;W[cf]CR[cf]
;B[af]CR[af]C[Black connects the ko and wins by one point! RIGHT]))
(;B[ad]CR[ad]
;W[cg]CR[cg]
;B[af]CR[af]
;W[fb]CR[fb]
(;B[gb]CR[gb]
;W[ae]CR[ae]C[White connects the ko - jigo!])
(;B[bg]CR[bg]
;W[gb]CR[gb]
;B[cf]CR[cf]
;W[dg]CR[dg]
;B[bf]CR[bf]
;W[ga]CR[ga]
;B[ea]CR[ea]C[White wins by one point!])))
(;B[ad]CR[ad]
;W[cf]CR[cf]
;B[af]CR[af]
;W[fb]CR[fb]
;B[gb]CR[gb]
;W[ae]CR[ae]C[Whites connects the ko - jigo!]))
(;B[ad]CR[ad]
;W[af]CR[af]C[This is bad - White wins by one point.])
(;B[cf]CR[cf]
;W[af]CR[af]
;B[ad]CR[ad]
;W[df]CR[df]C[This is bad - White wins by one point.])
(;B[bg]CR[bg]
;W[cg]CR[cg]C[Now black can't get a ko with a2 (Wa2 is good, too)]))