Theorem pm2.61

Description: Theorem *2.61 of WhiteheadRussell p. 107. Useful for eliminating an antecedent. (Contributed by NM, 4-Jan-1993) (Proof shortened by Wolf Lammen, 22-Sep-2013)

	Ref	Expression
Assertion	pm2.61	$$⊢\left(\phi \to \psi \right)\to \left(\left(\neg \phi \to \psi \right)\to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.6	$$⊢\left(\neg \phi \to \psi \right)\to \left(\left(\phi \to \psi \right)\to \psi \right)$$
2	1	com12	$$⊢\left(\phi \to \psi \right)\to \left(\left(\neg \phi \to \psi \right)\to \psi \right)$$