Theorem
2nexaln
Theorem *11.25 in [WhiteheadRussell] p. 160. (Contributed by Andrew Salmon, 24-May-2011.)
Conclusion
\( \vdash \neg \exists x \, \exists y \, \varphi \leftrightarrow \forall x \, \forall y \, \neg \varphi \)
Proof
2exnaln
bicomi
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con1bii
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