Endgame Problem 30 / Attempts

What is the largest play?  

yoyoma: Looks like starts a huge ko, I didn't look at the ko threat situation. The half point ko at a is the only other move I see.

Confused: Why can't Black answer at b instead of ?

yoyoma: Oops, I got lost in circular thinking. :-)

Another try  

Confused: I think forces with a net loss of a point for Black.

By the way...  

unkx80: By the way, if plays elsewhere, then lives unconditionally. Now a and b are miai.