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Normally a post like that is accompanied by a link to a Wikipedia article.
Normally a post like that is accompanied by a link to a Wikipedia article.
https://en.wikipedia.org/wiki/Interlock_(engineering) https://en.wikipedia.org/wiki/Interlock_(engineering)
True, sorry. User wpietry already posted the link, I just wanted to point out that the whole story is available there, which includes answer to his question.