Theorem velsn

Description: There is only one element in a singleton. Exercise 2 of TakeutiZaring p. 15. (Contributed by NM, 21-Jun-1993)

	Ref	Expression
Assertion	velsn	$$⊢x\in \left\{A\right\}\leftrightarrow x=A$$

Proof

Step	Hyp	Ref	Expression
1		vex	$$⊢x\in \mathrm{V}$$
2	1	elsn	$$⊢x\in \left\{A\right\}\leftrightarrow x=A$$