\begin{proposition}\label{assumetest}
For all $x$ we have $x = x$.
\end{proposition}
\begin{proof}
Fix $x$.
Then $x = x$.
\end{proof}
%\begin{proposition}\label{assumetesttwo}
% For all $x,y,z$ we have $z = z$.
%\end{proposition}
%\begin{proof}
% Fix $y$.
% Fix $x,z$.
% Then $z = z$.
%\end{proof}