>>296
Toward Useful Type-Free Theories. I
Solomon Feferman
The Journal of Symbolic Logic, Vol. 49, No. 1. (Mar., 1984), pp. 75-111.
http://www.impan.pl/~kz/truthseminar/Feferman_TypeFreeI.pdf

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> Thus in ZF there is no set of all sets, nor any Russell set {x | not(x ∈ x)} (which
> would be universal since ∀x(not x ∈ x) holds in ZF).

76ページ 下から4行目
> ZF and BG are both untyped formalisms, i.e. the levels which we have in mind in
> their informal interpretations do not appear explicitly in the syntax. Thus we can
> form expressions x ∈ y without restriction, in particular the expression x ∈ x.