Theorem
orbi1
Theorem *4.37 of [WhiteheadRussell] p. 118. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash (\varphi \leftrightarrow \psi) \rightarrow (\varphi \vee \chi \leftrightarrow \psi \vee \chi) \)
Proof
id
orbi1d
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