(;AB[rp][qp][pp][oo][on][nm][pm][mq]AW[qn][pn][ro][om][ol][pl][qk][rk]C[Black plays.

Source:
    A problem from the endgame volume of Gokyo Seimyo, a nearly 200 year old problem set.]SZ[19];B[rm]C[The best move!]

(;W[qm]C[White doesn't have time to capture the cutting stone here.];B[rn]C[Black's stones connect out, so White takes a relatively big loss.];W[qo];B[so])
(;W[rn]C[White has no choice but to fill an internal liberty.];B[qm]C[Black sacrifices yet another stone!];W[rl]LB[sm:A][ql:B]C[White can stay ahead in the capturing race for the 3 stones.  However, White has already had to play 2 stones within his own territory to match the 2 stones Black has sacrificed.  And later, White must lose 2 points playing at A and B to capture the 3 stones.

Black's endgame gain is modest, but could win a close game.]))