Theorem con2bid

Description: A contraposition deduction. (Contributed by NM, 15-Apr-1995)

	Ref	Expression
Hypothesis	con2bid.1	|- ( ph -> ( ps <-> -. ch ) )
Assertion	con2bid	|- ( ph -> ( ch <-> -. ps ) )

Proof

Step	Hyp	Ref	Expression
1		con2bid.1	|- ( ph -> ( ps <-> -. ch ) )
2		con2bi	|- ( ( ch <-> -. ps ) <-> ( ps <-> -. ch ) )
3	1 2	sylibr	|- ( ph -> ( ch <-> -. ps ) )