Class is Proper iff Bijection from Class to Proper Class/Corollary
Let $A$ be a class.
Let $\mathrm P$ be a proper class.
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From Biconditional is Transitive and Class is Proper iff Bijection from Class to Proper Class, it suffices to show that:
- There exists a bijection from $A$ to $\mathrm P$ iff there exists a bijection from $\mathrm P$ to $A$.
The rest follows from Inverse of Bijection is Bijection.