Theorem 2th

Description: Two truths are equivalent. (Contributed by NM, 18-Aug-1993)

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

Proof

Step	Hyp	Ref	Expression
1		2th.1	|- ph
2		2th.2	|- ps
3	2	a1i	|- ( ph -> ps )
4	1	a1i	|- ( ps -> ph )
5	3 4	impbii	|- ( ph <-> ps )