(;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]))))