Theorem
pm4.39
Theorem *4.39 of [WhiteheadRussell] p. 118. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash (\varphi \leftrightarrow \chi) \wedge (\psi \leftrightarrow \theta) \rightarrow (\varphi \vee \psi \leftrightarrow \chi \vee \theta) \)
Proof
l0
simpl
simpr
orbi12d
l0
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