Theorem
pm4.55
Theorem *4.55 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \neg (\neg \varphi \wedge \psi) \leftrightarrow \varphi \vee \neg \psi \)
Proof
pm4.54
con2bii
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bicomi
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