Shapes related to the carpenter's square
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Without outside liberties, White can kill by a ko.
The ko sequence. at a also leads to ko.
is wrong. After , and are miai, Black is dead.
cannot kill, the result is a seki. The reader may verify that at a does not work either.
White makes use of Black's shortage of liberties with and .
If makes an eye here, then is atari, which allows White to destroy the eye at the corner.
With outside liberties, White can still get a ko. (same as Gokyo Shumyo, Section 3, Problem 78).
Connecting underneath at allows to live without ko.
For a detailed analysis of this position, see L-group on the 4th line.