1. xander says:

    It is perhaps worth noting that “clopen” is also a term used in mathematics (specifically, in topology). While the abstract definitions are probably not that interesting, there are notions of “closed sets” and “open sets” in topology. Somewhat confusingly, these concepts are not mutually exclusive, leading to the possibility that a set can be both open and closed at the same time, i.e. clopen.

    There is an YouTube video that helps to explain the situation.

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