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Metamath Proof Explorer

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	$class A$
1	0	csn	$class \{A\}$
2		vx	$setvar x$
3	2	cv	$setvar x$
4	3 0	wceq	$wff x = A$
5	4 2	cab	$class \{x \| x = A\}$
6	1 5	wceq	$wff \{A\} = \{x \| x = A\}$