Ordinal Membership is Trichotomy/Proof 2

Proof
By Relation between Two Ordinals, it follows that:
 * $\paren {\alpha = \beta} \lor \paren {\alpha \subset \beta} \lor \paren {\beta \subset \alpha}$

By Transitive Set is Proper Subset of Ordinal iff Element of Ordinal, the result follows.