Black has a two-point gote (by miai counting) at a for later, which is why this is best. sacrifices one point to gain two.
I think White's ko threat at b is a better one than her ko threat at a in the above solution. Can someone verify that?
Bill: Yes, Wa is a losing ko threat.
Now black is the one with the ko threats, but still not as many points.
( fails, so white would not play this way.)
See also Korean Problem Academy/Errata.