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  1. \setlength\subproofhorizspace{1.1em}
  2. \begin{logicproof}{1}
  3. \forall a \forall b \; (P(a) \land Q(b)) & prem.\\
  4. \begin{subproof}
  5. \llap{$t\enspace \;$} \forall b \; (P(s) \land Q(b)) & $\forall \mathrm{e}$ 1\\
  6. P(s) \land Q(t) & $\forall \mathrm{e}$ 2\\
  7. P(s) & $\land \mathrm{e}_1$ 3\\
  8. P(s) \lor Q(t) & $\lor \mathrm{i}_1$ 4\\
  9. \exists b \; (P(s) \lor Q(b)) & $\exists \mathrm{i}$ 5
  10. \end{subproof}
  11. \forall a \exists b \; (P(a) \lor Q(b)) & $\forall \mathrm{i}$ 2-6
  12. \end{logicproof}