Definition:Class Equality

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

Then $A$ and $B$ are equal, denoted $A = B$, iff:


 * $\forall x: \left({ x \in A \iff x \in B }\right)$

where $\in$ denotes class membership.

When $x$ is a set variable, equality of $x$ and $A$ is defined using the same formula:


 * $x = A$ iff $\forall y: \left({y \in x \iff y \in A}\right)$
 * $A = x$ iff $\forall y: \left({y \in A \iff y \in x}\right)$

Comment
This definition "overloads" the $=$ sign, since $x = y$ could refer to either class equality or set equality.

However, this overloading is justified because for sets $x$ and $y$, $x = y$ is equal for either set equality or class equality.

This fact is proved on Class Equality Extension of Set Equality.

Also see

 * Class Equality Extension of Set Equality
 * Small Class