Theorem norasslem3

This lemma specializes biorf suitably for the proof of norass. (Contributed by Wolf Lammen, 18-Dec-2023.)

Conclusion
\( \vdash \neg \varphi \rightarrow (\psi \rightarrow \chi \leftrightarrow \varphi \vee \psi \rightarrow \chi) \)
Proof
biorf
imbi1d _