Gokyo Shumyo, Section 1, Problem 91 / Solution

Solution.  

The sequence of , and is important. After , Black suffering from a shortage of liberties, is unable to atari at a.

Failure.  

If White reverses the order of the moves , and , Black will capture two white stones with . Even when White captures a stone at and , it is still insufficient for White to live.

Failure.  

fails too. captures all the three White stones.