feat(set_theory/zfc/ordinal): more lemmas on transitive sets (#15548)
feat(set_theory/zfc/ordinal): more lemmas on transitive sets (#15548)

We add `empty_is_transitive`, `is_transitive.inter`, and `is_transitive.sUnion'`. We also add a `variables` block.





leanprover-community/mathlib,leanprover-community/mathlib