Theorem unexgOLD

Description: Obsolete version of unexg as of 21-Jul-2025. (Contributed by NM, 18-Sep-2006) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	unexgOLD	|- ( ( A e. V /\ B e. W ) -> ( A u. B ) e. _V )

Proof

Step	Hyp	Ref	Expression
1		elex	|- ( A e. V -> A e. _V )
2		elex	|- ( B e. W -> B e. _V )
3		unexb	|- ( ( A e. _V /\ B e. _V ) <-> ( A u. B ) e. _V )
4	3	biimpi	|- ( ( A e. _V /\ B e. _V ) -> ( A u. B ) e. _V )
5	1 2 4	syl2an	|- ( ( A e. V /\ B e. W ) -> ( A u. B ) e. _V )