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	$$⊢A\subseteq A$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢x\in A\to x\in A$$
2	1	ssriv	$$⊢A\subseteq A$$