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\setlength\subproofhorizspace{1.3em}
\begin{logicproof}{3}
\exists x \;(Q(y) \imp P(x)) & prem.\\
\begin{subproof}
\llap{$x_0\enspace \;$} Q(y) \imp P(x_0) & ass.\\
\begin{subproof}
Q(y) & ass.\\
P(x_0) & $\imp \mathrm{e}$ 3,2 \\
\exists x \; P(x) & $\exists \mathrm{i}$ 4
\end{subproof}
Q(y) \imp \exists x \; P(x) & $\imp \mathrm{i}$ 3-5
\end{subproof}
Q(y) \imp \exists x \; P(x) & $\exists \mathrm{e}$ 1, 2-6
\end{logicproof}