Theorem
pm2.68
Theorem *2.68 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash ((\varphi \rightarrow \psi) \rightarrow \psi) \rightarrow \varphi \vee \psi \)
Proof
jarl
orrd
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