Theorem
pm4.56
Theorem *4.56 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \neg \varphi \wedge \neg \psi \leftrightarrow \neg (\varphi \vee \psi) \)
Proof
ioran
bicomi
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