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  1. \setlength\subproofhorizspace{1.3em}
  2. \begin{logicproof}{4}
  3. \lnot \exists x \forall y \; (P(x) \land Q(y)) & \prem\\
  4. \begin{subproof}
  5. \begin{subproof}
  6. \forall y (P(x_0)\land Q(y)) & $\assum$ $\freshVar{$x_0$}$\\
  7. \exists x \forall y (P(x)\land Q(y)) & $\existi2$\\
  8. \bot & $\nege1,3$
  9. \end{subproof}
  10. \lnot \forall y (P(x_0)\land Q(y)) & $\negi2-4$\\
  11. \begin{subproof}
  12. \lnot \exists y \lnot (P(x_0)\land Q(y)) & $\assum$\\
  13. \begin{subproof}
  14. \begin{subproof}
  15. \lnot (P(x_0)\land Q(y_0)) & $\assum$ $\freshVar{$y_0$}$\\
  16. \exists y \lnot (P(x_0) \land Q(y)) & $\existi7$\\
  17. \bot & $\nege6,8$
  18. \end{subproof}
  19. P(x_0) \land Q(y_0) & $PBC 7-9$
  20. \end{subproof}
  21. \forall y (P(x_0)\land Q(y)) & $\foralli7-10$\\
  22. \bot & $\nege5,11$
  23. \end{subproof}
  24. \exists y \lnot (P(x_0)\land Q(y)) & $PBC 6-12$
  25. \end{subproof}
  26. \forall x \exists y \; \lnot (P(x) \land Q(y)) & $\foralli2-13$
  27. \end{logicproof}