Pairs, construction and reduction: https://www.youtube.com/watch?v=YEyyyzUuUJQ
Less than: Every domino half in this space must add up to less than the number.
。爱思助手下载最新版本对此有专业解读
H, C, N, O, F, P, S, Cl, Br, I
AAAAAAAAAAAAAAAAAAAAAA AAA A
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