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\setlength\subproofhorizspace{1em}
\begin{logicproof}{1}
\forall x \; (z=x) \land (f(x) = f(z)) & prem.\\
(z=y) \land f(y) = f(z) & $\forall \mathrm{e}$ 1\\
\begin{subproof}
y=z & ass.\\
f(y) = f(z) & $\land \mathrm{e}_2$2\\
\exists x \; f(y) = f(x) & $\exists \mathrm{i}$ 4
\end{subproof}
y = z \imp \exists x (f(y) = f(x)) & $\imp \mathrm{i}$ 3-5
\end{logicproof}