Alguien wrote:A15 connects.
I don't read b as you do. What if b goes directly to C12 and C9 instead of connecting?
A15 connects the upper 2 stones to the corner, but then other stones die (shown in the diagram originally posted).
If b goes directly to C12, I suppose that's a bit of a problem. I forgot to check that sequence. I feel like you might be able to push the stones into your thickness at the top and kill an even larger group as a result, but I guess the proverb goes something like "If you can't read exactly how the killing sequence plays out, you shouldn't attempt it".