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	|- ( ( ph -> ps ) -> ( ( -. ph -> ps ) -> ps ) )

Proof

Step	Hyp	Ref	Expression
1		pm2.6	|- ( ( -. ph -> ps ) -> ( ( ph -> ps ) -> ps ) )
2	1	com12	|- ( ( ph -> ps ) -> ( ( -. ph -> ps ) -> ps ) )