Theorem sstrid

Description: Subclass transitivity deduction. (Contributed by NM, 6-Feb-2014)

	Ref	Expression
Hypotheses	sstrid.1	|- A C_ B
	sstrid.2	|- ( ph -> B C_ C )
Assertion	sstrid	|- ( ph -> A C_ C )

Proof

Step	Hyp	Ref	Expression
1		sstrid.1	|- A C_ B
2		sstrid.2	|- ( ph -> B C_ C )
3	1	a1i	|- ( ph -> A C_ B )
4	3 2	sstrd	|- ( ph -> A C_ C )