Theorem pm2.621

Description: Theorem *2.621 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.621	$$⊢\left(\phi \to \psi \right)\to \left(\left(\phi \vee \psi \right)\to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢\left(\phi \to \psi \right)\to \left(\phi \to \psi \right)$$
2		idd	$$⊢\left(\phi \to \psi \right)\to \left(\psi \to \psi \right)$$
3	1 2	jaod	$$⊢\left(\phi \to \psi \right)\to \left(\left(\phi \vee \psi \right)\to \psi \right)$$