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Re:Set Theory and Foundations of Mathematics by defen on Friday July 31, 2015 @01:32PM (#9983032)
There is no U in ZF so I really don't understand what you are saying.
Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @01:57PM (#9983218)
You can call U anything you want. It's by axiom schema of specification: for any set A, there some set B with a set C in B iff C in A.