Definition df-sn

Description: Define the singleton of a class. Definition 7.1 of Quine p. 48. For convenience, it is well-defined for proper classes, i.e., those that are not elements of _V , see snprc . For an alternate definition see dfsn2 . (Contributed by NM, 21-Jun-1993)

	Ref	Expression
Assertion	df-sn	|- { A } = { x | x = A }

Detailed syntax breakdown

Step	Hyp	Ref	Expression
0		cA	|- A
1	0	csn	|- { A }
2		vx	|- x
3	2	cv	|- x
4	3 0	wceq	|- x = A
5	4 2	cab	|- { x | x = A }
6	1 5	wceq	|- { A } = { x | x = A }