Dieter: I know the solution now, but I want to discuss here why I find this problem difficult.
- It has a starting width of 6 (for me): 6 moves I can't dismiss without reading. a and b narrow the eyespace, while all of c, d, e and f look like vital points
- The 3rd move is a blind spot
If we play the two reducing moves, then White lives. This shows two vital points.
here doesn't work either but already hints at the solution
If we try , we fail again
This is ko and looks like the best Black can do.
So, I couldn't solve this problem and would definitely misjudge the status here as 'Black can make a ko'.
But black can kill. We have found but is the blind spot.