Axiom:Axiom of Extension/Set Theory/Formulation 2

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

The  can be formulated as:
 * $\forall x: \paren {\paren {A = B \land A \in x} \implies B \in x}$

This formulation is used in set theories that define $=$ instead of admitting it as a primitive.