(;AB[rq]AB[qq]AB[pr]AB[ss]AW[or]AW[oq]AW[pq]AW[qp]AW[qo]AW[qn]AW[qm]AW[mr]TR[ss]C[How the marked stone works?]AP[goproblems]
(;B[rr];W[ps]
(;B[qs]C[Well done. Ko is the best result. The marked stone worked bad for black.RIGHT]TR[ss])
(;B[qr];W[qs])
(;B[rp];W[qr]
(;B[ro];W[rn]
(;B[so];W[sq])
(;B[sq];W[so]))
(;B[sq];W[ro])))
(;B[qs];W[rr]
(;B[sr];W[rp])
(;B[rp];W[qr]))
(;B[ps];W[rr]
(;B[sr];W[rp]
(;B[qr];W[rs])
(;B[rs];W[qr]))
(;B[qr];W[rs]
(;B[sr];W[rp])
(;B[rp];W[ro]
(;B[sp];W[sr])
(;B[sr];W[sp])))
(;B[rs];W[qr])
(;B[rp];W[qr]))
(;B[qr];W[rp]
(;B[ps];W[sq]
(;B[sr];W[rs])
(;B[rs];W[sr]))
(;B[sq];W[ps]
(;B[qs];W[rr])
(;B[rr];W[qs]))
(;B[rr];W[sq])
(;B[rs];W[ps]))
(;B[rs];W[ps])
(;B[sr];W[qr]
(;B[rr];W[qs])
(;B[qs];W[ps])
(;B[ps];W[qs]))
(;B[os];W[qr]
(;B[rr];W[ps])
(;B[ps];W[qs])
(;B[qs];W[rr]
(;B[sr];W[rp])
(;B[rp];W[ro]
(;B[sr];W[sp])
(;B[sp];W[so]))))
(;B[rp];W[qr]
(;B[rr];W[qs])
(;B[ps];W[qs])
(;B[qs];W[ps])))