Theorem notnotd

Description: Deduction associated with notnot and notnoti . (Contributed by Jarvin Udandy, 2-Sep-2016) Avoid biconditional. (Revised by Wolf Lammen, 27-Mar-2021)

	Ref	Expression
Hypothesis	notnotd.1	$$⊢\phi \to \psi$$
Assertion	notnotd	$$⊢\phi \to \neg \neg \psi$$

Proof

Step	Hyp	Ref	Expression
1		notnotd.1	$$⊢\phi \to \psi$$
2		notnot	$$⊢\psi \to \neg \neg \psi$$
3	1 2	syl	$$⊢\phi \to \neg \neg \psi$$