Beginner Exercise 252 / Solution

Not a ko-threat.  

is not a ko-threat.

Variation 1.  

If , then blocks. Then if , then is an atari, and then connects.

togo: This obviously only works because there is an additional freedom at a.

Variation 2.  

The tricky move is . However, solves the problem. Subsequently, if White a cuts the Black stones, then the atari at b transposes back to Variation 1.

Variation 2, Black mistake.  

The Black move at is a mistake. Now, after , Black cannot avoid the ko at a. Even if pushes, can block, and then a Black move at b will be self-atari, so the ko still cannot be avoided.