Theorem onun2i

Description: The union of two ordinal numbers is an ordinal number. (Contributed by NM, 13-Jun-1994)

	Ref	Expression
Hypotheses	on.1	|- A e. On
	on.2	|- B e. On
Assertion	onun2i	|- ( A u. B ) e. On

Proof

Step	Hyp	Ref	Expression
1		on.1	|- A e. On
2		on.2	|- B e. On
3		onun2	|- ( ( A e. On /\ B e. On ) -> ( A u. B ) e. On )
4	1 2 3	mp2an	|- ( A u. B ) e. On