(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PB[Black to play]
AB[qq][pq][qp][qo][np][nm][mm][kn][lm][jo][jp][kq][lr][mr][ol][ql][rm]
AW[oq][op][pp][pn][no][on][mo][lo][lp][mq][nr]
C[This is the problem position. Black to play.]
(;B[po]C[If black tries to stop the upper eye...]
;W[pr]
;B[ns]
;W[os]
;B[ms]
;W[qr]
;B[rr]
;W[qs]C[.. then white easily gets an eye on the edge.])
(;B[pr]C[If black tries to stop the edge eye..]
;W[po]
;B[or]
;W[nq]C[... then the other two eyes are made.])
(;B[nq]C[Adding an extra stone instead is the way to stop
two eyes forming. This is correct.]
(;W[mp]C[If white takes the two stones...]
;B[nq]C[... black must play in again]
;W[pr]
;B[ns]
(;W[or]
;B[po]C[This way white dies.])
(;W[os]C[So white had better play here to start a ko.]
;B[or]
;W[pc]
;B[pd]
;W[nr]C[This is white's best response.]
;B[qd]
;W[ms]))
(;W[pr]
;B[ns]
;W[os]
;B[or]C[Again a ko])
(;W[os]
;B[pr]
;W[po]
;B[or]
;W[mp]
;B[nq]C[This way white dies.])))
