Theorem eq0ALT

Description: Alternate proof of eq0 . Shorter, but requiring df-clel , ax-8 . (Contributed by NM, 29-Aug-1993) Avoid ax-11 , ax-12 . (Revised by GG and Steven Nguyen, 28-Jun-2024) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	eq0ALT	$$⊢A=\varnothing \leftrightarrow \forall x\neg x\in A$$

Proof

Step	Hyp	Ref	Expression
1		dfcleq	$$⊢A=\varnothing \leftrightarrow \forall x\left(x\in A\leftrightarrow x\in \varnothing \right)$$
2		noel	$$⊢\neg x\in \varnothing$$
3	2	nbn	$$⊢\neg x\in A\leftrightarrow \left(x\in A\leftrightarrow x\in \varnothing \right)$$
4	3	albii	$$⊢\forall x\neg x\in A\leftrightarrow \forall x\left(x\in A\leftrightarrow x\in \varnothing \right)$$
5	1 4	bitr4i	$$⊢A=\varnothing \leftrightarrow \forall x\neg x\in A$$