(;CA[gb2312]AB[jc][qc][nb][nc][oe][ne][id][mf][pf][qf][rf][og][oh][me][pi][qi][ri]
[rj][kd]AW[oc][pd][pe][re][rd][of][nf][ng][mg][pg][lf][lh][mi][ni][mj][nk][oj][pj]
[qj][rk][kh][jg][je][ic][ob]CR[qa][qb][qc][rc][rb][ra][sa][sb][sc][sd]C[*Problem extracted from "Thirty-Six Stratagems" by Xiaochun Ma. For the comeplete game review please refer to stratagem #4 in the book.

Black to play. What are some of the end game strategies that can be used in the corner?
]
AP[MultiGo:4.4.4]SZ[19]AB[oi]MULTIGOGM[1]

(;B[rc]C[Variation 1];W[ra];B[qb];W[qa];B[oa];W[pa];B[na];W[pb];B[od];W[pc];B[sb]
;W[rb];B[sf];W[qd]C[Black 1 to test white's response. 

From the exchanges in the corner we can see that black had taken both sides, 7,13, and 9 with a sente. 
White has a total of 10 points in the corner.])
(;B[rb]C[Variation 2];W[pb];B[od];W[pc];B[qb];W[rc];B[se];W[sb];B[sd];W[qa];B[oa]
;W[qd]C[If black decides to play at 1, then white exchanges with 2, 4, 6, 8, and 10 which are mandatory. 

Comparing to variation 1, in the end white only makes 8 points instead of 10.])
(;B[pb]
(;W[oa];B[rc];W[pa];B[qa];W[pc];B[rb];W[qb]C[If white 2 decides to cut black's connection, the corner results in a approach ko.])
(;W[pc];B[oa];W[qb];B[qa];W[rb];B[ra];W[rc];B[sb];W[sc];B[se];W[pa];B[pb]C[White can choose to avoid the approach ko by playing Q17, but will be suffering much from the end game.])))