Theorem cbvalv1

Description: Rule used to change bound variables, using implicit substitution. Version of cbval with a disjoint variable condition, which does not require ax-13 . See cbvalvw for a version with two more disjoint variable conditions, requiring fewer axioms, and cbvalv for another variant. (Contributed by NM, 13-May-1993) (Revised by BJ, 31-May-2019)

	Ref	Expression
Hypotheses	cbvalv1.nf1	⊢ Ⅎ 𝑦 𝜑
	cbvalv1.nf2	⊢ Ⅎ 𝑥 𝜓
	cbvalv1.1	⊢ ( 𝑥 = 𝑦 → ( 𝜑 ↔ 𝜓 ) )
Assertion	cbvalv1	⊢ ( ∀ 𝑥 𝜑 ↔ ∀ 𝑦 𝜓 )

Proof

Step	Hyp	Ref	Expression
1		cbvalv1.nf1	⊢ Ⅎ 𝑦 𝜑
2		cbvalv1.nf2	⊢ Ⅎ 𝑥 𝜓
3		cbvalv1.1	⊢ ( 𝑥 = 𝑦 → ( 𝜑 ↔ 𝜓 ) )
4	3	biimpd	⊢ ( 𝑥 = 𝑦 → ( 𝜑 → 𝜓 ) )
5	1 2 4	cbv3v	⊢ ( ∀ 𝑥 𝜑 → ∀ 𝑦 𝜓 )
6	3	biimprd	⊢ ( 𝑥 = 𝑦 → ( 𝜓 → 𝜑 ) )
7	6	equcoms	⊢ ( 𝑦 = 𝑥 → ( 𝜓 → 𝜑 ) )
8	2 1 7	cbv3v	⊢ ( ∀ 𝑦 𝜓 → ∀ 𝑥 𝜑 )
9	5 8	impbii	⊢ ( ∀ 𝑥 𝜑 ↔ ∀ 𝑦 𝜓 )