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>正則性公理があると、帰納法の議論が、簡単になるのも事実だなｗ

正則性公理：Given the other axioms of Zermelo?Fraenkel set theory, the axiom of regularity is equivalent to the axiom of induction.
つまりは、ZF上で、正則性公理と帰納法公理は、同値だと

https://en.wikipedia.org/wiki/Axiom_of_regularity
Axiom of regularity
(抜粋)
In mathematics, the axiom of regularity (also known as the axiom of foundation) is an axiom of Zermelo?Fraenkel set theory that states that every non-empty set A contains an element that is disjoint from A.

The axiom of regularity was introduced by von Neumann (1925); it was adopted in a formulation closer to the one found in contemporary textbooks by Zermelo (1930). Virtually all results in the branches of mathematics based on set theory hold even in the absence of regularity; see chapter 3 of Kunen (1980).
However, regularity makes some properties of ordinals easier to prove; and it not only allows induction to be done on well-ordered sets but also on proper classes that are well-founded relational structures such as the lexicographical ordering on {(n,α ) ｜ n ∈ ω ∧ α is an ordinal }.

Given the other axioms of Zermelo?Fraenkel set theory, the axiom of regularity is equivalent to the axiom of induction.
The axiom of induction tends to be used in place of the axiom of regularity in intuitionistic theories (ones that do not accept the law of the excluded middle), where the two axioms are not equivalent.

In addition to omitting the axiom of regularity, non-standard set theories have indeed postulated the existence of sets that are elements of themselves.

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