Theorem ssun3

Description: Subclass law for union of classes. (Contributed by NM, 5-Aug-1993)

	Ref	Expression
Assertion	ssun3	$$⊢A\subseteq B\to A\subseteq B\cup C$$

Proof

Step	Hyp	Ref	Expression
1		ssun1	$$⊢B\subseteq B\cup C$$
2		sstr2	$$⊢A\subseteq B\to \left(B\subseteq B\cup C\to A\subseteq B\cup C\right)$$
3	1 2	mpi	$$⊢A\subseteq B\to A\subseteq B\cup C$$