Theorem notnotriALT

Description: Alternate proof of notnotri . The proof via notnotr and ax-mp also has three essential steps, but has a total number of steps equal to 8, instead of the present 7, because it has to construct the formula ph twice and the formula -. -. ph once, whereas the present proof has to construct the formula ph twice and the formula -. ph once, and therefore makes only one use of wn instead of two. This can be checked by running the Metamath command "MM> SHOW PROOF notnotri / NORMAL". (Contributed by NM, 27-Feb-2008) (Proof shortened by Wolf Lammen, 15-Jul-2021) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Hypothesis	notnotri.1	$$⊢\neg \neg \phi$$
Assertion	notnotriALT	$$⊢\phi$$

Proof

Step	Hyp	Ref	Expression
1		notnotri.1	$$⊢\neg \neg \phi$$
2	1	pm2.21i	$$⊢\neg \phi \to \phi$$
3	2	pm2.18i	$$⊢\phi$$