Theorem
pm4.67
Theorem *4.67 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \neg (\neg \varphi \rightarrow \neg \psi) \leftrightarrow \neg \varphi \wedge \psi \)
Proof
pm4.63
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