Theorem
imbi1
Theorem *4.84 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
Conclusion
\( \vdash (\varphi \leftrightarrow \psi) \rightarrow (\varphi \rightarrow \chi \leftrightarrow \psi \rightarrow \chi) \)
Proof
id
imbi1d
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