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  1. \setlength{\subproofhorizspace}{1.5em}
  2. \begin{logicproof}{3}
  3. \lnot \exists x~P(x) \lor \lnot \exists y~Q(y) &prem \\
  4. \begin{subproof}
  5. \begin{subproof}
  6. \hspace*{-2.75em}
  7. \llap{$z_0\enspace \;$} \: \hspace{2.75em} Q(z_0) \land P(z_0) &assum \\
  8. \begin{subproof}
  9. \lnot \exists x~P(x) & assum \\
  10. P(z_0) &$\land \mathrm{e} 2$ \\
  11. \exists x~P(x) &$\exists \mathrm{i} 4$ \\
  12. \bot &$\lnot \mathrm{e} 3,5$
  13. \end{subproof}
  14. \begin{subproof}
  15. \lnot \exists y~Q(y) & assum \\
  16. Q(z_0) &$\land \mathrm{e} 2$ \\
  17. \exists y~Q(y) &$\exists \mathrm{i} 8$ \\
  18. \bot &$\lnot \mathrm{e} 7,9$
  19. \end{subproof}
  20. \bot &$\lor \mathrm{e} 1,3-6,7-10$
  21. \end{subproof}
  22. \lnot(Q(z_0) \land P(z_0)) &$\lnot \mathrm{i} 3-11$
  23. \end{subproof}
  24. \forall z \; \lnot(Q(z) \land P(z)) &$\forall \mathrm{i} 3-12$
  25. \end{logicproof}