Gokyo Shumyo, Section 1, Problem 42 / Solution
is the vital point here, making eyes at the marked points.
If Black captures two stones ...
... White recaptures and her eyes are still real ones.
It is tempting to protect two stones, but Black will block at a or b. White can at most extend to six stones on the second line, and as we know "six die but eight live".
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