Theorem con3i

Description: A contraposition inference. Inference associated with con3 . Its associated inference is mto . (Contributed by NM, 3-Jan-1993) (Proof shortened by Wolf Lammen, 20-Jun-2013)

	Ref	Expression
Hypothesis	con3i.a	|- ( ph -> ps )
Assertion	con3i	|- ( -. ps -> -. ph )

Proof

Step	Hyp	Ref	Expression
1		con3i.a	|- ( ph -> ps )
2		id	|- ( -. ps -> -. ps )
3	2 1	nsyl	|- ( -. ps -> -. ph )