Theorem rexeqOLD

Description: Equality theorem for restricted existential quantifier. (Contributed by NM, 29-Oct-1995) Remove usage of ax-10 , ax-11 , and ax-12 . (Revised by Steven Nguyen, 30-Apr-2023) (New usage is discouraged.) (Proof modification is discouraged.)

	Ref	Expression
Assertion	rexeqOLD	⊢ ( 𝐴 = 𝐵 → ( ∃ 𝑥 ∈ 𝐴 𝜑 ↔ ∃ 𝑥 ∈ 𝐵 𝜑 ) )

Proof

Step	Hyp	Ref	Expression
1		biidd	⊢ ( 𝐴 = 𝐵 → ( 𝜑 ↔ 𝜑 ) )
2	1	rexeqbi1dv	⊢ ( 𝐴 = 𝐵 → ( ∃ 𝑥 ∈ 𝐴 𝜑 ↔ ∃ 𝑥 ∈ 𝐵 𝜑 ) )