Theorem
notnotb
Double negation. Theorem *4.13 of [WhiteheadRussell] p. 117. (Contributed by NM, 3-Jan-1993.)
Conclusion
\( \vdash \varphi \leftrightarrow \neg \neg \varphi \)
Proof
l0
notnot
notnotr
impbii
l0
_
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