Theorem 19.3vOLD

Description: Obsolete version of 19.3v as of 20-Oct-2023. (Contributed by Anthony Hart, 13-Sep-2011) Remove dependency on ax-7 . (Revised by Wolf Lammen, 4-Dec-2017) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	19.3vOLD	$$⊢\forall x\phi \leftrightarrow \phi$$

Proof

Step	Hyp	Ref	Expression
1		alex	$$⊢\forall x\phi \leftrightarrow \neg \exists x\neg \phi$$
2		19.9v	$$⊢\exists x\neg \phi \leftrightarrow \neg \phi$$
3	2	con2bii	$$⊢\phi \leftrightarrow \neg \exists x\neg \phi$$
4	1 3	bitr4i	$$⊢\forall x\phi \leftrightarrow \phi$$