(;SZ[19]AP[MultiGo:2.8.0]AB[ca][cb][cc][bd][ad]AW[cd][dc][db][da][be][ce][ae]
TR[ae]C[white played the mark stone. If you think the group is safe tenuki at A.]LB[fe:A]
(;B[bb] ;W[ab](;B[aa]
;W[ba]C[This is a ko. The correct solution has black living unconditionally.]
)
(;B[ac]
;W[bc]C[Oops.]
)
(;B[bc]
;W[aa]C[Seki...
no wait \:)]LB[fe:A]
;B[fe] ;W[ba] ;B[ac]
;W[aa]C[Dead]
)
)
(;B[ab]C[Sneaky \:)]
(;W[ba]C[CHOICE]
(;B[bb] ;W[bc]
;B[ac]C[Alive.
A18 was the key move. It does allow white 2 consecutive ko threats but if white got to go A18 the group would be dead.
RIGHT]
)
(;B[aa]; W[bc]
(;B[bb]; W[ac]
)
(;B[ac]; W[bb]
)
)
(;B[bc]C[Black lives
RIGHT]
)
)
(;W[bb]C[CHOICE]
(;B[ba] ;W[bc]
;B[ac]C[Alive.
A18 was the key move. It does allow white 2 consecutive ko threats but if white got to go A18 the group would be dead.
RIGHT]
)
(;B[bc]C[Alive!
RIGHT]
)
)
(;W[bc]C[CHOICE]
;B[ac](;W[bb]C[CHOICE]
;B[ba]C[Alive.
A18 was the key move. It does allow white 2 consecutive ko threats but if white got to go A18 the group would be dead.
RIGHT]
)
(;W[ba]C[CHOICE]
;B[bb]C[Alive.
A18 was the key move. It does allow white 2 consecutive ko threats but if white got to go A18 the group would be dead.
RIGHT]
)
)
)
(;B[fe](;W[bc]C[CHOICE]
;B[bb]
;W[ac]C[Dead]
)
(;W[bb]C[CHOICE]
(;B[bc]
;W[ba]C[Dead]
)
(;B[ab]
;W[bc]C[Dead]
)
)
(;W[ab]C[CHOICE]
;B[bb]
;W[ac]C[Dead]
)
)
)