Theorem pm5.21

Two propositions are equivalent if they are both false. Theorem *5.21 of [WhiteheadRussell] p. 124. (Contributed by NM, 21-May-1994.)

Conclusion
\( \vdash \neg \varphi \wedge \neg \psi \rightarrow (\varphi \leftrightarrow \psi) \)
Proof
pm5.21im
imp _