Theorem n0

Description: A class is nonempty if and only if it has at least one element. Proposition 5.17(1) of TakeutiZaring p. 20. (Contributed by NM, 29-Sep-2006)

	Ref	Expression
Assertion	n0	$$⊢A\ne \varnothing \leftrightarrow \exists xx\in A$$

Proof

Step	Hyp	Ref	Expression
1		nfcv	$$⊢\underset{\_}{Ⅎ}xA$$
2	1	n0f	$$⊢A\ne \varnothing \leftrightarrow \exists xx\in A$$