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% Try this example or insert your own text. Axiom. There is no $y$ such that $y \in \emptyset$. Define $x$ to be transitive if and only if for all $u$, $v$, if $u \in v$ and $v \in x$ then $u\in x$. Define $x$ to be an ordinal if and only if $x$ is transitive and for all $y$, if $y \in x$ then $y$ is transitive. Then $\emptyset$ is an ordinal.
