Theorem
pm2.5
Theorem *2.5 of [WhiteheadRussell] p. 107. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash \neg (\varphi \rightarrow \psi) \rightarrow \neg \varphi \rightarrow \psi \)
Proof
pm2.5g
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