User:Dfeuer/Definition:Unordered Pair
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Definition
Let $a$ and $b$ be sets.
Then the unordered pair $\{ a, b \}$ is the class of all sets $x$ such that $x = a$ or $x = b$.
Remark
Such a class exists by the User:Dfeuer/Axiom Schema of Separation.
That class is unique by the User:Dfeuer/Axiom of Extensionality.