Gokyo Shumyo, Section 2, Problem 2 / Solution
Traditional solution
kills the group without ko. This line works in the corrected version too.
a and b become miai for the kill. This is inferior to the main line because white has more ko threats.
After white is left with the bulky 5 shape.
Alternatives
However Black has three more moves to kill: a to c.
For example, this line kills because White can't connect at a.