(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2] RU[Japanese]SZ[19]KM[0.00] AW[po][qo][ro][so][pp][qq][pr][qr][qs]AB[rk][ml][pl][ql][sl][lm][nm][om][rm][mn][on][pn][qn][rn][sn][mo][oo][mp][op][mq][oq][pq][kr][mr][or][ms][os][ps]LB[nn:a][no:b][np:c][nq:d][nr:e][ns:f]C[What is the status of the corner? (Make no assumptions about the rest of the board.) (a) White is dead (tenuki) (b) Black can kill (c) Black can force a ko for life (d) Black can force white to live in seki (avoiding ko fight) (e) White is living with two eyes (tenuki) (f) None of the above FORCE] (;B[nn]) (;B[no] ;W[np]C[Kill the white group!] AB[np] (;B[rq] ;W[rp] (;B[rr] ;W[rs] (;B[sr] ;W[sp]C[White lives in seki.]) (;B[sq] ;W[ss]C[White lives in seki.])) (;B[sq] ;W[rr] ;B[sr] ;W[rs]C[White lives in seki.]) (;B[sr] ;W[rr] ;B[sq] ;W[rs]C[White lives in seki.])) (;B[rp] ;W[rq] (;B[sp] ;W[qp]) (;B[sq] ;W[sr]))) (;B[np] ;W[no]C[Start a ko for life!] AB[no] (;B[rq] ;W[rp] (;B[rr] ;W[rs] (;B[sr] ;W[sp]C[White lives in seki.]) (;B[sq] ;W[ss]C[White lives in seki.])) (;B[sq] ;W[rr] ;B[sr] ;W[rs]C[White lives in seki.]) (;B[sr] ;W[rr] ;B[sq] ;W[rs]C[White lives in seki.])) (;B[rp] ;W[rq] (;B[sp] ;W[qp]) (;B[sq] ;W[sr]))) (;B[nq] ;W[nr]C[Correct, white can live in seki for sure. But if white has much better ko threats than black, then white can start a ko and get some territory in the corner. Click 'N' (O4) to switch colors! FORCE] AB[nr]LB[np:N] ;B[np] AW[rk][ml][pl][ql][sl][lm][nm][om][rm][mn][on][pn][qn][rn][sn][mo][oo][mp][np][op][mq][nq][oq][pq][kr][mr][nr][or][ms][os][ps]AB[po][qo][ro][so][pp][qq][pr][qr][qs] ;W[rq]C[Now black does not settle with seki but wants the whole corner. Find the best ko!] (;B[rp] ;W[rr] (;B[rs] ;W[sr] ;B[sq]C[RIGHT]) (;B[sq] ;W[rs])) (;B[rr] ;W[sp] ;B[qp] ;W[sr] ;B[ss]C[Two stage ko. Black can do better!]) (;B[sq] ;W[sr] (;B[rp] ;W[sp]C[Disadv. ko, but white has an internal ko threat at S2. Black can do better!]) (;B[sp] ;W[qp]) (;B[qp] ;W[sp] (;B[rp] ;W[sq]) (;B[rr] ;W[ss]C[Seki.])) (;B[rr] ;W[sp] ;B[qp] ;W[ss]C[Seki.])) (;B[sp] ;W[sq] ;B[qp] ;W[rr]) (;B[qp] ;W[sp] (;B[rr] ;W[sr] ;B[ss]C[Two stage ko. Black can do better!]) (;B[rp] ;W[rr] ;B[sq] ;W[rs]) (;B[sr] ;W[rr] ;B[rs]C[Seki.])) (;B[sr] ;W[rr] (;B[rs] ;W[sp] ;B[qp]C[Seki.]) (;B[rp] ;W[rs]) (;B[sq] ;W[rs]))) (;B[nr] ;W[nq]C[Click 'N' (O5) to switch colors. FORCE] AB[nq]LB[no:N] ;B[no] AW[rk][ml][pl][ql][sl][lm][nm][om][rm][mn][on][pn][qn][rn][sn][mo][no][oo][mp][op][mq][nq][oq][pq][kr][mr][nr][or][ms][os][ps]AB[po][qo][ro][so][pp][qq][pr][qr][qs] ;W[rq]C[Try to live with two eyes!] (;B[rp] ;W[rr] (;B[rs] ;W[sr] ;B[sp]C[The corner lives in seki.]) (;B[sq] ;W[sr] ;B[rs] ;W[sp]C[Ko.])) (;B[qp] ;W[rr]) (;B[rr] ;W[sp] ;B[qp] ;W[sr] ;B[rs]C[The corner lives in seki.]) (;B[sq] ;W[sr] (;B[rp] ;W[sp]) (;B[rr] ;W[sp] ;B[qp] ;W[rs]C[Ko.]))) (;B[ns] ;W[nr]C[Correct! White can live in seki for sure. But if white has good enough ko threats, it can also start a ko and get some territory in the corner. Click 'N' (O4) to switch colors! FORCE] AB[nr]LB[np:N] ;B[np] AW[rk][ml][pl][ql][sl][lm][nm][om][rm][mn][on][pn][qn][rn][sn][mo][oo][mp][np][op][mq][oq][pq][kr][mr][nr][or][ms][ns][os][ps]AB[po][qo][ro][so][pp][qq][pr][qr][qs] ;W[rq]C[Now show how to live in seki!] (;B[rp] ;W[rr] (;B[rs] ;W[sr] ;B[sp]C[RIGHT]) (;B[sq] ;W[sr] (;B[rs] ;W[sp]C[Ko.]) (;B[ss] ;W[sp]C[Ko.]) (;B[qp] ;W[sp]C[Ko.]))) (;B[rr] ;W[sp] ;B[qp] ;W[sr] ;B[rs]C[RIGHT]) (;B[sq] ;W[rr] (;B[sr] ;W[rs] ;B[qp] ;W[sp]) (;B[rp] ;W[sr] ;B[rs] ;W[sp]C[Ko.]) (;B[sp] ;W[rs] ;B[qp] ;W[rp]) (;B[rs] ;W[sr] ;B[rp] ;W[sp]C[Ko.])) (;B[sp] ;W[sq] ;B[qp] ;W[rr]) (;B[qp] ;W[sp] (;B[rr] ;W[sr] ;B[rs]C[RIGHT]) (;B[rp] ;W[rr] ;B[sq] ;W[rs]) (;B[sr] ;W[rr] ;B[rs]C[RIGHT])) (;B[sr] ;W[rr] (;B[rs] ;W[sq] ;B[rp] ;W[ss]C[Ko.]) (;B[rp] ;W[rs]) (;B[sq] ;W[rs]))))