Theorem axfrege41

Description: Identical to notnot . Axiom 41 of Frege1879 p. 47. (Contributed by RP, 24-Dec-2019)

	Ref	Expression
Assertion	axfrege41	|- ( ph -> -. -. ph )

Proof

Step	Hyp	Ref	Expression
1		notnot	|- ( ph -> -. -. ph )