Endgame Problem 4 / Solution

Black to play  

This position is hyperactive. Correct play depends on the ko threat situation.

White komaster  

When White is komaster saves the stones. Later, - is sente. (Somewhat sneakily, Wa instead is also sente. ;-)) Then a, b, c, and d are miai, played as dame or ko threats.

Black komaster  

When Black is komaster he can play more enterprisingly, with - .

Black komaster (ii)  

That leaves behind a large throw-in ko at . But Black takes first with , forcing . Now saves his stones. White still has a sente ko with - Ba. (Since Black is komaster, White will avoid this ko.)