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The "Formal presentation" section begin with a description of the Axiom of extensionality. However I don't see how this axiom is relevant nor that it is used anywhere in the rest of the section, which just uses
Comprehension, Existential instantiation and Universal instantiation. Extentionality is
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Its definition is like the like the set R in the article: i.e. x belongs to R if and only if x does not belong to x (however, determine whether a positive integer x is a
Russell number may be very difficult, the smallest unknown x is 319)
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also not mentioned in the source cited. Should the reference to extentionality be deleted?
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