My 30th problem! Yay! Thanks to kevinwm for helping me add more paths.
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SZ [4]
AB [ac][bd][cc][dc]
AW [aa][bb][bc][cd]
C [Black to play and get the best result.]
(; B [cb] (; W [ad] (; B [ca] (; W [bd] (; B [ba] (; W [db] (; B [ab]; W [aa] (; B [ab] C [RIGHT])(; B [da]; W [ab]))(; B [da]; W [ab]))(; W [ab]; B [dd]; W [db]; B [ac] C [RIGHT][Another choice for white; the computer will not choose to play this line.]))(; B [dd]; W [db]))(; W [ab]; B [dd]; W [db]; B [bd] C [RIGHT][Another choice for white; the computer will not choose to play this line.])(; W [db]; B [bd] C [RIGHT][Another choice for white; the computer will not choose to play this line.]))(; B [dd]; W [bd]; B [ca]; W [db])(; B [da]; W [ab])(; B [ba]; W [ab] (; B [ca]; W [db])(; B [dd]; W [ca]; B [bd]; W [db] (; B [ac]; W [cd])(; B [da]; W [cd]; B [ba]))))(; W [ca]; B [dd]; W [da] C [RIGHT][Seki, black wins by one point (the prisoner). This is best for both.]))
(; B [dd]; W [cb]; B [db]; W [ca])
(; B [ca]; W [cb] (; B [db]; W [ad])(; B [dd]; W [db]))
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