The play at is the vital point.
The points a and b are miai now.
xela: What is black's response to ?
xela: this is the best I can find so far...
I can almost see how black can make a ko, but it looks like white has enough liberties to squeeze... have I missed something here?
unkx80: Given your , the refutation is easy - and turns it into a direct ko.
For , then and kills without ko. If this is the case, then this really improves over your suggested .
There is a short discussion at /Failure.