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:
评选方式：所有入围作品将公布于稍后发布的少数派活动专页，少数派注册用户届时均可投票。每个账号最多可投票一次，每次最多选三个作品。最终结果完全由用户票数决定。但少数派有权依自主判断，取消异常投票或违规作品的资格。
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But of course, like any immutable system, there are mutable parts (otherwise, we couldn’t create any configuration files). OSTree handles this with “overlays” (actually, we use OverlayFS) that allow a read-write filesystem to be layered on top of the immutable system. For example, the /etc and /var directories are writable, while the rest of the system is read-only.
第四十五条 以营利为目的，提供漏洞探测、渗透性测试等服务的机构，应当向设区的市级以上公安机关备案。