User:Dfeuer/Set has Unique Singleton
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Theorem
Let $x$ be a set.
Then $x$ has a unique User:Dfeuer/Definition:Singleton.
Proof
By the User:Dfeuer/Axiom Schema of Separation, there is a class $S$ such that
- $\forall y: y \in S \iff y = x$
By the User:Dfeuer/Axiom of Extensionality, this class is unique.
$\blacksquare$