Theorem con4d

Description: Deduction associated with con4 . (Contributed by NM, 26-Mar-1995)

	Ref	Expression
Hypothesis	con4d.1	$$⊢\phi \to \left(\neg \psi \to \neg \chi \right)$$
Assertion	con4d	$$⊢\phi \to \left(\chi \to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		con4d.1	$$⊢\phi \to \left(\neg \psi \to \neg \chi \right)$$
2		con4	$$⊢\left(\neg \psi \to \neg \chi \right)\to \left(\chi \to \psi \right)$$
3	1 2	syl	$$⊢\phi \to \left(\chi \to \psi \right)$$