Theorem
xor3
Two ways to express "exclusive or". (Contributed by NM, 1-Jan-2006.)
Conclusion
\( \vdash \neg (\varphi \leftrightarrow \psi) \leftrightarrow (\varphi \leftrightarrow \neg \psi) \)
Proof
pm5.18
con2bii
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bicomi
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