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つづき

https://en.wikipedia.org/wiki/Axiom_of_regularity
Axiom of regularity

Regularity in the presence of urelements
Urelements are objects that are not sets, but which can be elements of sets. In ZF set theory, there are no urelements, but in some other set theories such as ZFA, there are. In these theories, the axiom of regularity must be modified. The statement "{\displaystyle x\not =\emptyset }{\displaystyle x\not =\emptyset }" needs to be replaced with a statement that {\displaystyle x}x is not empty and is not an urelement. One suitable replacement is {\displaystyle (\exists y)[y\in x]}{\displaystyle (\exists y)[y\in x]}, which states that x is inhabited.
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