Theorem pm5.13

Theorem *5.13 of [WhiteheadRussell] p. 123. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 14-Nov-2012.)

Conclusion

\( \vdash (\varphi \rightarrow \psi) \vee (\psi \rightarrow \varphi) \)

Proof

pm5.14