Beginner Exercise 229 / Solution

Solution  

White should simply retreat at , not allowing Black to play a in sente. Further crawling is futile - for example blocks safely. So Black dies as explained in the L-group page.

Alternative Solution  

In practice, attachment can be a good alternative. Black will need another move at a to separate the marked stones.

Comparison  

After in the "Solution" diagram, Black will play , limiting the corner territory and separating the marked stones in sente.

Failure  

Immediately blocking with is a big mistake. After , a making life and b connecting to outside are miai.