Theorem mt2d

Description: Modus tollens deduction. (Contributed by NM, 4-Jul-1994)

	Ref	Expression
Hypotheses	mt2d.1	|- ( ph -> ch )
	mt2d.2	|- ( ph -> ( ps -> -. ch ) )
Assertion	mt2d	|- ( ph -> -. ps )

Proof

Step	Hyp	Ref	Expression
1		mt2d.1	|- ( ph -> ch )
2		mt2d.2	|- ( ph -> ( ps -> -. ch ) )
3	2	con2d	|- ( ph -> ( ch -> -. ps ) )
4	1 3	mpd	|- ( ph -> -. ps )