This is actually an endgame problem.
secures his territory. Later - are an ambiguous exchange. Black gets 9 points of territory.
fills at .
puts the four stones in atari. Later - are sente. Black gets 3 points and White gets 8 points, for a net local score of -5. The original position has a count of +2, and a play gains 7 points.
If White starts with , may reply without ko. Then Black cannot connect after .
Black can still play for ko with , but then White can use a as a ko threat, or play there to capture if Black plays at b without making ko.