This is a ko. Assuming is a ko threat and black answers with
then black cannot kill by playing under the stones as shown in the next two diagrams.
If Black replys to ko threat at then White takes ko and Black has to find the big ko threat to force White's response and then retake ko.
If Black ignores the ko threat and takes with , then
.
If @ b then
@ c,
and if
attacks on top at e or f, then
at a captures the four Black stones with the double cut; and if
connects on the bottom then
at f gets the second eye.
If on top at d, e, or f, then
still at c, Black connects on the bottom at D-1 or F-1 to protect against the double cut, and
at b makes White's second eye. (In the case of
at e white may fill to protect
and threaten the cut at f making f and b miai for the second eye.)
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