Theorem orbi12i

Description: Infer the disjunction of two equivalences. (Contributed by NM, 3-Jan-1993)

	Ref	Expression
Hypotheses	orbi12i.1	|- ( ph <-> ps )
	orbi12i.2	|- ( ch <-> th )
Assertion	orbi12i	|- ( ( ph \/ ch ) <-> ( ps \/ th ) )

Proof

Step	Hyp	Ref	Expression
1		orbi12i.1	|- ( ph <-> ps )
2		orbi12i.2	|- ( ch <-> th )
3	2	orbi2i	|- ( ( ph \/ ch ) <-> ( ph \/ th ) )
4	1	orbi1i	|- ( ( ph \/ th ) <-> ( ps \/ th ) )
5	3 4	bitri	|- ( ( ph \/ ch ) <-> ( ps \/ th ) )