Definition:Class Equality/Definition 2
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Let $A$ and $B$ be classes.
$A$ and $B$ are equal, denoted $A = B$, if and only if:
- $A \subseteq B$ and $B \subseteq A$
where $\subseteq$ denotes the subclass relation.
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $2$: Some Basics of Class-Set Theory: $\S 1$ Extensionality and separation