Ideas from 'works' by Ernst Zermelo [1920], by Theme Structure

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4. Formal Logic / F. Set Theory ST / 4. Axioms for Sets / a. Axioms for sets
Zermelo made 'set' and 'member' undefined axioms [Chihara]
For Zermelo's set theory the empty set is zero and the successor of each number is its unit set [Blackburn]