Theorem pm5.501

Theorem *5.501 of [WhiteheadRussell] p. 125. (Contributed by NM, 3-Jan-2005.)

Conclusion
\( \vdash \varphi \rightarrow (\psi \leftrightarrow (\varphi \leftrightarrow \psi)) \)
Proof
l0
pm5.1im
biimp
com12 _
impbid l0 _