Theorem
pm4.66
Theorem *4.66 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \neg \varphi \rightarrow \neg \psi \leftrightarrow \varphi \vee \neg \psi \)
Proof
pm4.64
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