Theorem con2bii

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

	Ref	Expression
Hypothesis	con2bii.1	$$⊢\phi \leftrightarrow \neg \psi$$
Assertion	con2bii	$$⊢\psi \leftrightarrow \neg \phi$$

Proof

Step	Hyp	Ref	Expression
1		con2bii.1	$$⊢\phi \leftrightarrow \neg \psi$$
2		notnotb	$$⊢\psi \leftrightarrow \neg \neg \psi$$
3	2 1	xchbinxr	$$⊢\psi \leftrightarrow \neg \phi$$