Theorem pm2.73

Description: Theorem *2.73 of WhiteheadRussell p. 108. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.73	$$⊢\left(\phi \to \psi \right)\to \left(\left(\left(\phi \vee \psi \right)\vee \chi \right)\to \left(\psi \vee \chi \right)\right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.621	$$⊢\left(\phi \to \psi \right)\to \left(\left(\phi \vee \psi \right)\to \psi \right)$$
2	1	orim1d	$$⊢\left(\phi \to \psi \right)\to \left(\left(\left(\phi \vee \psi \right)\vee \chi \right)\to \left(\psi \vee \chi \right)\right)$$