Swelled Class contains Empty Set
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Theorem
Let $A$ be a swelled class.
Then the empty set is an element of $A$.
Proof
By definition of swelled class, every subclass of every element of $A$ is also an element of $A$.
Let $x \in A$.
Then by Empty Class is Subclass of All Classes, the empty class is an element of $A$.
By the Axiom of the Empty Set, the empty class is a set.
Hence the result.
$\blacksquare$