Theorem ssid

Description: Any class is a subclass of itself. Exercise 10 of TakeutiZaring p. 18. (Contributed by NM, 21-Jun-1993) (Proof shortened by Andrew Salmon, 14-Jun-2011)

	Ref	Expression
Assertion	ssid	|- A C_ A

Proof

Step	Hyp	Ref	Expression
1		id	|- ( x e. A -> x e. A )
2	1	ssriv	|- A C_ A