Metamath Proof Explorer

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 x x A


Step Hyp Ref Expression
1 nfcv _ x A
2 1 n0f A x x A