Theorem expi

Description: An exportation inference. (Contributed by NM, 29-Dec-1992) (Proof shortened by Mel L. O'Cat, 28-Nov-2008)

	Ref	Expression
Hypothesis	expi.1	$$⊢\neg \left(\phi \to \neg \psi \right)\to \chi$$
Assertion	expi	$$⊢\phi \to \left(\psi \to \chi \right)$$

Proof

Step	Hyp	Ref	Expression
1		expi.1	$$⊢\neg \left(\phi \to \neg \psi \right)\to \chi$$
2		pm3.2im	$$⊢\phi \to \left(\psi \to \neg \left(\phi \to \neg \psi \right)\right)$$
3	2 1	syl6	$$⊢\phi \to \left(\psi \to \chi \right)$$