%% 
%%  Example 02-02-16 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\[
 x=\begin{cases}
        0 & \text{if A=...}\\
        1 & \text{if B=...}\\
        x & \textrm{this runs with as much text as you like, but without a linebreak,
                    it runs out of page....}
   \end{cases}
\]
\end{document}
