(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PB[Black to play]C[Just play through the moves to move 18 which is the problem position. These do not represent a game sequence!]
;B[bq]
;W[bp]
;B[cp]
;W[bo]
;B[co]
;W[cn]
;B[dp]
;W[do]
;B[cr]
;W[dq]
;B[ap]
;W[bn]
;B[]
;W[eo]
;B[]
;W[fq]
;B[]
;W[hq]C[This is the problem position. Black to play.]
(;B[dr]C[This looks like a threat]
;W[er]C[but white can just block]
;B[bs]
;W[ar]C[An there is no room for two eyes in the corner.])
(;B[bs]
;W[ar]C[This prevents the corner eyes]
;B[er]
;W[eq]
;B[dr]C[This makes an on on the edge]
;W[aq]C[But this leaves black with only one eye.])
(;B[er]C[This is a better threat against white]
(;W[cq]C[White can cut the three stones]
;B[dr]
;W[eq]C[But this is forced by black]
;B[bs]C[And now the move on B1 guarantees an
eye in the corner and one on the edge.

This is the correct sequence.])
(;W[eq]
;B[dr]C[If white doesn't take the chance to cut the three
stones, black still plays here.]
;W[ar]
;B[aq]
;W[fr]
;B[es]C[But there is still enough room to make a live
shape in the corner and black may get 
to save the three stones later.]
;W[cq]
;B[br]C[White cutting may be sente but loses ko threats
against the corner.])))
