Theorem rexsngOLD

Description: Obsolete version of rexsng as of 30-Sep-2024. (Contributed by NM, 29-Jan-2012) (Proof shortened by AV, 7-Apr-2023) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Hypothesis	ralsngOLD.1	$$⊢x=A\to \left(\phi \leftrightarrow \psi \right)$$
Assertion	rexsngOLD	$$⊢A\in V\to \left(\exists x\in \left\{A\right\}\phi \leftrightarrow \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		ralsngOLD.1	$$⊢x=A\to \left(\phi \leftrightarrow \psi \right)$$
2		nfv	$$⊢Ⅎx\psi$$
3	2 1	rexsngf	$$⊢A\in V\to \left(\exists x\in \left\{A\right\}\phi \leftrightarrow \psi \right)$$