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There is no ko.
allows for a ko.
is the vital point, black can answer at , or a, but it all fails. Note that works because Black has no external liberties (Black would put the group in atari if he tries to make a second eye by playing at b).
If Black does have an external liberty, however, would not work. In that case, White can only go for a ko.
The attack used for the "no external liberties" case is repeated here to show that an unconditional kill is not possible. Since there is one external liberty, Black can safely play to make a second eye.
Here is an example of a sequence that leads to ko. Although White does take first, Black may be able to use a as a local ko threat if the escape to the center is possible.