Theorem
bibi1
Theorem *4.86 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash (\varphi \leftrightarrow \psi) \rightarrow (\varphi \leftrightarrow \chi \leftrightarrow (\psi \leftrightarrow \chi)) \)
Proof
id
bibi1d
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