Theorem negbi 87

Description: Infer double negation.

Hypothesis

Ref	Expression
negbi.1	|- ph

Assertion

Ref	Expression
negbi	|- -. -. ph

Proof of Theorem negbi

Step	Hyp	Ref	Expression
1		negbi.1	. 2 |- ph
2		negb 86	. 2 |- (ph -> -. -. ph)
3	1, 2	ax-mp 7	1 |- -. -. ph

Syntax hints:  -. wn 2

This theorem is referenced by:  nbn3 721  dm0 3312  dmsn0 3313  dmsnsn0 3314

This theorem was proved from axioms:  ax-1 4  ax-2 5  ax-3 6  ax-mp 7

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