a,b,and c are the stronger responses to .
After , Black can't get both x and y
After , White has x to escape or y to capture some black stones by snapback.
Capturing with only delays the problem; at is good enough.
The purpose of is to prevent the white chain with from escaping. If at x, still works.
Compare with Resistance 'c' the extra stone at allows Black to fight a ko.
See also a simpler variant at Xuanxuan Qijing Problem 112.