(;AB[qr]AB[qq]AB[pq]AB[oq]AB[op]AB[lp]AB[lq]AB[lr]AB[ls]AW[ks]AW[kr]AW[kq]AW[kp]AW[ko]AW[lo]AW[mo]AW[no]AW[oo]AW[pp]AW[qp]AW[rq]AW[rr]AW[ro]AW[nq]C[Black to live.]AP[goproblems]
(;B[nr];W[mq]
(;B[mr]
(;W[ps]C[CHOICE]
(;B[qs];W[ns]
(;B[or];W[os]C[Seki.RIGHT])
(;B[os];W[or];B[pr];W[os]))
(;B[os];W[qs]))
(;W[qs]C[Wrong white move. Black is alive with points.];B[ps]
(;W[or];B[os])
(;W[rs];B[or])
(;W[ns];B[or])))
(;B[os];W[mr]
(;B[pr];W[qs])
(;B[qs];W[pr]))
(;B[ps];W[mr]
(;B[or];W[ns])
(;B[ns];W[or])))
(;B[np]C[Pushing at O4 reduces black liberties. In some situations this is a bad idea...];W[mp]
(;B[nr];W[mq];B[mr];W[qs];B[ps];W[or]
(;B[pr];W[os])
(;B[rs];W[ns])
(;B[ns];W[pr]))
(;B[mq];W[nr]TR[ls]TR[lr]TR[lq]TR[lp]TR[mq]C[The tirangle stones are captured.]))
(;B[or];W[nr];B[np];W[mp]
(;B[mq];W[ms])
(;B[ns];W[mq]))
(;B[mq];W[nr];B[np];W[mp])
(;B[os];W[nr];B[np];W[mp];B[mq];W[ms]))