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  1. \setlength\subproofhorizspace{1.6em}
  2. \begin{logicproof}{1}
  3. \exists x \; \lnot P(x) & prem.\\
  4. \forall x \; \lnot Q(x) & prem.\\
  5. \begin{subproof}
  6. \llap{$x_0\enspace \;$} \lnot P(x_0) & ass.\\
  7. \lnot Q(x_0) & $\forall \mathrm{e}$ 2\\
  8. \lnot P(x_0) \land \lnot Q(x_0) & $\land \mathrm{i}$ 3,4\\
  9. \exists x \; (\lnot P(x) \land \lnot Q(x)) & $\exists \mathrm{i}$ 5
  10. \end{subproof}
  11. \exists x \; (\lnot P(x) \land \lnot Q(x)) & $\exists \mathrm{e}$ 3-6
  12. \end{logicproof}