Theorem pm2.37

Description: Theorem *2.37 of WhiteheadRussell p. 105. (Contributed by NM, 6-Mar-2008)

	Ref	Expression
Assertion	pm2.37	$$⊢\left(\psi \to \chi \right)\to \left(\left(\psi \vee \phi \right)\to \left(\phi \vee \chi \right)\right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.38	$$⊢\left(\psi \to \chi \right)\to \left(\left(\psi \vee \phi \right)\to \left(\chi \vee \phi \right)\right)$$
2		pm1.4	$$⊢\left(\chi \vee \phi \right)\to \left(\phi \vee \chi \right)$$
3	1 2	syl6	$$⊢\left(\psi \to \chi \right)\to \left(\left(\psi \vee \phi \right)\to \left(\phi \vee \chi \right)\right)$$