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