Theorem
pm2.54
Theorem *2.54 of [WhiteheadRussell] p. 107. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash (\neg \varphi \rightarrow \psi) \rightarrow \varphi \vee \psi \)
Proof
df-or
biimpri
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