Bent four in the corner exercises / Solution 3

White to play  

ChrisSchack: is the vital point, and kills unconditionally.

Black tries to extend  

If B tries to get more eyespace, W simply seals him in, and W cannot be prevented from playing a, then killing with an almost-fill by a bent four at the end of the game. This would only be ko if White needed to capture before game-end, to ensure a surrounding group's safety.