Gokyo Shumyo, Section 1, Problem 50 / Solution
Black's formation is a carpenter's square. The vital point of this shape is the marked point (2-2 point?). If White plays there instead of at the marked stone, the position becomes ko.
Yohei: When all the outside liberties are filled, as is in this case, White a or b also make ko.
Go to Gokyo Shumyo, Section 1, Problem 51
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