Theorem ioossre

Description: An open interval is a set of reals. (Contributed by NM, 31-May-2007)

	Ref	Expression
Assertion	ioossre	|- ( A (,) B ) C_ RR

Proof

Step	Hyp	Ref	Expression
1		elioore	|- ( x e. ( A (,) B ) -> x e. RR )
2	1	ssriv	|- ( A (,) B ) C_ RR