And, although it is difficult to find references to knocking wood for good luck before the nineteenth century, it turns out that this is not the case for touching iron — the “tocca ferro” of Italy, shown above.
缺点：容易饱和（输入过大或过小时梯度接近0，导致梯度消失）
，这一点在WPS官方版本下载中也有详细论述
中国援冈比亚农业技术合作项目组供图
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