Theorem pm2.18dOLD

Description: Obsolete version of pm2.18d as of 17-Nov-2023. (Contributed by FL, 12-Jul-2009) (Proof shortened by Andrew Salmon, 7-May-2011) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Hypothesis	pm2.18dOLD.1	$$⊢\phi \to \left(\neg \psi \to \psi \right)$$
Assertion	pm2.18dOLD	$$⊢\phi \to \psi$$

Proof

Step	Hyp	Ref	Expression
1		pm2.18dOLD.1	$$⊢\phi \to \left(\neg \psi \to \psi \right)$$
2		pm2.18OLD	$$⊢\left(\neg \psi \to \psi \right)\to \psi$$
3	1 2	syl	$$⊢\phi \to \psi$$