Axiom:Axiom of Extension/Class Theory

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

Then:
 * $\forall x: \paren {x \in A \iff x \in B} \iff A = B$

Hence the order in which the elements are listed in the classes is immaterial.

Also see

 * Definition:Class Equality
 * Definition:Equals


 * Axiom:Axiom of Extension (Sets)