(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
AW[ca][bb][cb][cc][ad][bd][cd]AB[da][ab][db][ac][dc][dd][ae][be][ce][de]LB[gd:A]C[Black to kill.]
(;B[bc]
;W[aa])
(;B[ba]
;W[aa]C[Ko, white takes first. You can do better.])
(;B[aa]C[Right! This isn't a seki; it's bent-four in the corner. After removing all white's ko threats, black can kill at B19.RIGHT])
(;B[gd]
;W[aa]
;B[ba]C[Ko, white takes first. You can do better.]))