(;AB[rs]AB[rr]AB[rq]AB[qq]AB[pr]AW[qr]AW[rp]AW[qp]AW[pq]AW[oq]AW[nr]AW[po]C[White to kill.]AP[goproblems] (;W[ps];B[qs] (;W[os]C[RIGHT]) (;W[sq];B[os]) (;W[or];B[os])) (;W[sq];B[sr] (;W[ps];B[qs];W[os]C[From a life-and-death perspective, this variation also kills. However, professionals and dan players would not exchange A for B. It is sub-optimal, but the reasons why are subtle. In this case, there are two reasons. First, should the outside white stones get involved in a capturing race so that white needs to capture black's corner, black can capture at D, forcing white E, increasing black's liberties and creating a ko mouth, which means potential for trouble. Second, black F becomes a threat to bridge under with a ko. Without the A and B exchange, there is no potential for black F to be a sente move.RIGHT]LB[sq:A]LB[sr:B]LB[ps:C]LB[sp:D]LB[ro:E]LB[rn:F]) (;W[qs];B[ps])) (;W[sr];B[sq] (;W[ps];B[qs];W[os]C[RIGHT]) (;W[qs];B[ps])) (;W[qs];B[ps]) (;W[or];B[ps]) (;W[os];B[ps]) (;W[sp];B[ps]))