Theorem
pm3.1
Theorem *3.1 of [WhiteheadRussell] p. 111. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \varphi \wedge \psi \rightarrow \neg (\neg \varphi \vee \neg \psi) \)
Proof
anor
biimpi
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