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Lets assume, we would double, triple, quadruple, etc. Bobs world and stack these copies on top of each other. Bob would never notice, as long as each copy is just a “shallow” copy, meaning that each movement in the fundamental domain (the original world) is mirrored in the copies. In this case, we would have a covering space of Bobs world, and the original world would be the base space.
，这一点在51吃瓜中也有详细论述
Наука и техника。谷歌浏览器下载对此有专业解读
then the += will modify the list, and then raise an exception because you can’t assign to that tuple item because the tuple is immutable, but the list will have been mutated.