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%%  Example 02-09-3 
%%  Copyright (C) 2016 Herbert Voss
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\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\begin{subequations}
\renewcommand\theequation{\theparentequation -\arabic{equation}}%
\begin{align}
y &= d                  \\
y &= cx+d               \\
y &= bx^{2}+cx+d        \\\label{eq:subequation-neu}
y &= ax^{3}+bx^{2}+cx+d
\end{align}
\end{subequations}
A reference to eq.~\ref{eq:subequation-neu}
\end{document}
