Theorem pm2.49

Description: Theorem *2.49 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.49	$$⊢\neg \left(\phi \vee \psi \right)\to \left(\neg \phi \vee \neg \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.46	$$⊢\neg \left(\phi \vee \psi \right)\to \neg \psi$$
2	1	olcd	$$⊢\neg \left(\phi \vee \psi \right)\to \left(\neg \phi \vee \neg \psi \right)$$