White to kill  

If , white must play , a hard move to find.

Alex: I don't know if that's really the hardest move to find in this sequence. and are a standard tesuji combination for winning capturing races/creating a shortage of liberties in the corner, since they effectively make an eye. Once you've played , what else could you think of playing? Actually, can anyone provide a link to another problem that uses the same - combination? Groups of problems that employ similar tactics are more useful than isolated problems, I think, since then you're more likely to recognise similar patterns in a real game.