What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
if (n <= 1) return;
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Number (4): Everything in this space must add up to 4. The answer is 5-3, placed vertically; 1-1, placed horizontally.，详情可参考快连下载安装
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