Theorem mtbird

Description: A deduction from a biconditional, similar to modus tollens. (Contributed by NM, 10-May-1994)

	Ref	Expression
Hypotheses	mtbird.min	|- ( ph -> -. ch )
	mtbird.maj	|- ( ph -> ( ps <-> ch ) )
Assertion	mtbird	|- ( ph -> -. ps )

Proof

Step	Hyp	Ref	Expression
1		mtbird.min	|- ( ph -> -. ch )
2		mtbird.maj	|- ( ph -> ( ps <-> ch ) )
3	2	biimpd	|- ( ph -> ( ps -> ch ) )
4	1 3	mtod	|- ( ph -> -. ps )