Theorem 2thALT

Description: Alternate proof of 2th . (Contributed by Hongxiu Chen, 29-Jun-2025) (Proof modification is discouraged.) (New usage is discouraged.)

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

Proof

Step	Hyp	Ref	Expression
1		2thALT.1	$$⊢\phi$$
2		2thALT.2	$$⊢\psi$$
3		pm5.1im	$$⊢\phi \to \left(\psi \to \left(\phi \leftrightarrow \psi \right)\right)$$
4	1 2 3	mp2	$$⊢\phi \leftrightarrow \psi$$