%% 
%%  Example 03-03-12 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{134.31393pt}

%StartShownPreambleCommands

%StopShownPreambleCommands
\begin{document}
$\underline{x}+\underline{y}=\underline{z}$\\
\let\ul\underline
\def\yPh{\vphantom{y}}
$ \ul{x\yPh} + \ul{y} = \ul{z\yPh} $\\
$ \ul{x_1} + \ul{y_2} = \ul{z_3} $
\end{document}
