Theorem imor

Implication in terms of disjunction. Theorem *4.6 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-1993.)

Conclusion
\( \vdash \varphi \rightarrow \psi \leftrightarrow \neg \varphi \vee \psi \)
Proof
notnotb
l1
imbi1i _
\( \vdash \varphi \rightarrow \psi \leftrightarrow \neg \neg \varphi \rightarrow \psi \)
df-or
\( \vdash \neg \varphi \vee \psi \leftrightarrow \neg \neg \varphi \rightarrow \psi \)
bitr4i l1 _