is the key move that aims at the cut at a and effectively stops from escaping.
At any time, White can make the exchange of x/y or y/x , but the rest of the solution stays the same.
We will take a look at White's responses a, b , and c.
After , White can't get both x and y.
In fact, at y would probably be better, but this diagram also explains the variation when Black plays before .
After , White can't get both x and y.
Herman Hiddema: What if white plays at z, making x ko and defending the cutting point y at the same time?
char: I am not sure =p.
Herman Hiddema: Black can't even escape I think. Difficult problem :-)
char: I will take a look at it again tomorrow after I get some sleep. In the mean time, if you are sure that this doesn't work, feel free to change the solution.
char: Does that meant this doesn't work either?
Herman Hiddema: I think so. I've thought long and hard about it, but I haven't found a good reply to this (or to in the other variant)