For sets x and y, a class belongs to exactly when it is a function from x to y, and is a set, being a subclass of the power set of x×y.
In the setting of Class Theory NBG: the Axioms, Standing Conventions and Basic Notation, let and be sets.
For every class , is an element of the class of functions from to if and only if is a function from to , .
The class is a set.
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