Theorem con2bii

Description: A contraposition inference. (Contributed by NM, 12-Mar-1993)

	Ref	Expression
Hypothesis	con2bii.1	|- ( ph <-> -. ps )
Assertion	con2bii	|- ( ps <-> -. ph )

Proof

Step	Hyp	Ref	Expression
1		con2bii.1	|- ( ph <-> -. ps )
2		notnotb	|- ( ps <-> -. -. ps )
3	2 1	xchbinxr	|- ( ps <-> -. ph )