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Class**

Within Zermelo-Fraenkel set theory (and other axiomatisations), a class is a collection of sets satisfying some predicate, for example \(\{x|R(x)\}\), where \(R\) is a predicate which identifies \(x\) as describing a ring. Classes are often not sets as they are 'too large', in which case the class is called 'proper' - the aforementioned collection of all rings is an example of a proper class.