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I love the book Lessons in the Fundamentals of Go, but I have never understood chapter 3.
Here's a bit of a refresher. He begins with this diagram:
$$ -------------------------
$$ . . . . . . . . . . . .-
$$ . . . . . . . . . . . .-
$$ . . 2 . . X . O . . . .-
$$ . . , . . . . . , X . .-
$$...