Theorem ssun4

Description: Subclass law for union of classes. (Contributed by NM, 14-Aug-1994)

	Ref	Expression
Assertion	ssun4	$$⊢A\subseteq B\to A\subseteq C\cup B$$

Proof

Step	Hyp	Ref	Expression
1		ssun2	$$⊢B\subseteq C\cup B$$
2		sstr2	$$⊢A\subseteq B\to \left(B\subseteq C\cup B\to A\subseteq C\cup B\right)$$
3	1 2	mpi	$$⊢A\subseteq B\to A\subseteq C\cup B$$