Metamath Proof Explorer


Theorem bj-nnfbd

Description: If two formulas are equivalent for all x , then nonfreeness of x in one of them is equivalent to nonfreeness in the other, deduction form. See bj-nnfbi . (Contributed by BJ, 27-Aug-2023)

Ref Expression
Hypothesis bj-nnfbd.1 ( 𝜑 → ( 𝜓𝜒 ) )
Assertion bj-nnfbd ( 𝜑 → ( Ⅎ' 𝑥 𝜓 ↔ Ⅎ' 𝑥 𝜒 ) )

Proof

Step Hyp Ref Expression
1 bj-nnfbd.1 ( 𝜑 → ( 𝜓𝜒 ) )
2 1 alrimiv ( 𝜑 → ∀ 𝑥 ( 𝜓𝜒 ) )
3 bj-nnfbi ( ( ( 𝜓𝜒 ) ∧ ∀ 𝑥 ( 𝜓𝜒 ) ) → ( Ⅎ' 𝑥 𝜓 ↔ Ⅎ' 𝑥 𝜒 ) )
4 1 2 3 syl2anc ( 𝜑 → ( Ⅎ' 𝑥 𝜓 ↔ Ⅎ' 𝑥 𝜒 ) )