Tapir: It is a ko, white will ignore any threat but black will profit somewhere else.
Herman Hiddema: Correct. Bonus question: What is the biggest ko threat on the board for black?
Tapir: a is worth 14 points. (6 captured stones, 6 points of territory, 1 rescued stone and one point less white territory. That is the biggest one I see.
Herman Hiddema: Correct again. That was also the one played in the game. End result: W+0.5