Theorem pm2.21ddALT

Description: Alternate proof of pm2.21dd . (Contributed by Mario Carneiro, 9-Feb-2017) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Hypotheses	pm2.21ddALT.1	|- ( ph -> ps )
	pm2.21ddALT.2	|- ( ph -> -. ps )
Assertion	pm2.21ddALT	|- ( ph -> ch )

Proof

Step	Hyp	Ref	Expression
1		pm2.21ddALT.1	|- ( ph -> ps )
2		pm2.21ddALT.2	|- ( ph -> -. ps )
3	2	pm2.21d	|- ( ph -> ( ps -> ch ) )
4	1 3	mpd	|- ( ph -> ch )