Axiom:Axiom of Extension/Set Theory/Formulation 1

Axiom
Let $A$ and $B$ be sets.

The axiom of extension can be formulated as:
 * $\forall x: \paren {x \in A \iff x \in B} \iff A = B$

Axiom of Extension for Classes
The axiom of extension in the context of class theory has the same form: