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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:，这一点在WPS下载最新地址中也有详细论述
At least 11 killed after military plane carrying banknotes crashes in Bolivia，更多细节参见safew官方版本下载
优点：输出在 (−1,1)，比 sigmoid 居中，对梯度更友好。业内人士推荐WPS下载最新地址作为进阶阅读