Theorem 2th

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

	Ref	Expression
Hypotheses	2th.1	$$⊢\phi$$
	2th.2	$$⊢\psi$$
Assertion	2th	$$⊢\phi \leftrightarrow \psi$$

Proof

Step	Hyp	Ref	Expression
1		2th.1	$$⊢\phi$$
2		2th.2	$$⊢\psi$$
3	2	a1i	$$⊢\phi \to \psi$$
4	1	a1i	$$⊢\psi \to \phi$$
5	3 4	impbii	$$⊢\phi \leftrightarrow \psi$$