Theorem con4bid

Description: A contraposition deduction. (Contributed by NM, 21-May-1994)

	Ref	Expression
Hypothesis	con4bid.1	|- ( ph -> ( -. ps <-> -. ch ) )
Assertion	con4bid	|- ( ph -> ( ps <-> ch ) )

Proof

Step	Hyp	Ref	Expression
1		con4bid.1	|- ( ph -> ( -. ps <-> -. ch ) )
2	1	biimprd	|- ( ph -> ( -. ch -> -. ps ) )
3	2	con4d	|- ( ph -> ( ps -> ch ) )
4	1	biimpd	|- ( ph -> ( -. ps -> -. ch ) )
5	3 4	impcon4bid	|- ( ph -> ( ps <-> ch ) )