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+\documentclass[DIV=9, pagesize=auto]{scrartcl}
+
+\usepackage{fixltx2e}
+\usepackage{lmodern}
+\usepackage[T1]{fontenc}
+\usepackage{textcomp}
+\usepackage{amsmath}
+\usepackage{cases}
+\usepackage{microtype}
+\usepackage{hyperref}
+
+\newcommand*{\fancybreak}{%
+  \par
+  \nopagebreak\medskip\nopagebreak
+  \noindent\null\hfill$*\quad*\quad*\quad$\hfill\null\par
+  \nopagebreak\medskip\pagebreak[0]%
+}
+\newcommand*{\mail}[1]{\href{mailto:#1}{\texttt{#1}}}
+\newcommand*{\pkg}[1]{\textsf{#1}}
+\newcommand*{\cmd}[1]{\texttt{\string#1}}
+\newcommand*{\env}[1]{\texttt{#1}}
+\newcommand*{\meta}[1]{\textlangle\textsl{#1}\textrangle}
+
+\addtokomafont{title}{\rmfamily}
+
+\title{The \pkg{cases} package\thanks{This manual corresponds to \pkg{cases}~v2.5, dated~May 2002.}}
+\author{Donald Arseneau\\\mail{asnd@triumf.ca}}
+\date{May 2002}
+
+
+\begin{document}
+
+\maketitle
+
+\begingroup
+\small
+\noindent
+Copyright \textcopyright~1993, 1994, 1995, 2000, 2002 by Donald Arseneau, \mail{asnd@triumf.ca}.
+These macros may be freely transmitted, reproduced, or modified 
+provided that this notice is left intact.  Sub-equation numbering
+is based on \pkg{subeqn.sty} by Stephen Gildea; most of the rest is based
+on \LaTeX's \cmd{\eqnarray} by Leslie Lamport and the \LaTeX3 team.
+\par
+\endgroup
+
+\fancybreak
+
+This provides a \LaTeX\ environment \verb+{numcases}+ to produce multi-case 
+equations with a separate equation number for each case.  There is
+also \verb+{subnumcases}+ which numbers each case with the overall equation
+number plus a letter [8a, 8b, etc.].   The syntax is
+%
+\begin{verbatim}
+\begin{numcases}{left_side}
+  case_1 & explanation_1 \\
+  case_2 & explanation_2 \\
+  ...
+  case_n & explanation_n
+\end{numcases}
+\end{verbatim}
+%
+Each \meta{case} is a math formula, and each \meta{explanation} is a piece of lr mode
+text (which may contain math mode in \verb+\(...\)+ or \verb+$...$+).  The explanations
+are optional.  Equation numbers are inserted automatically, just as for
+the \env{eqnarray} environment.  In particular, the \cmd{\nonumber} command suppresses
+an equation number and the \cmd{\label} command allows reference to a particular 
+case.  In a \env{subnumcases} environment, a \cmd{\label} in the \meta{left\_side} of the 
+equation gives the overall equation number, without any letter. 
+
+To use this package, 
+include  ``\verb+\usepackage{cases}+''  after ``\cmd{\documentclass}''.  You may also
+specify  ``\verb+\usepackage[subnum]{cases}+''  to force \emph{all} \env{numcases}
+environments to be treated as \env{subnumcases}.
+
+\emph{Question:} Is there a \verb+{numcases*}+ environment for
+unnumbered cases?\\
+\emph{Answer:} There is a \verb+{cases}+
+environment in \AmS-\LaTeX, but it is just as convenient to stick
+with the canonical \LaTeX\ array:
+% 
+\begin{verbatim}
+\[ left side = \left\{ \begin{array}...\end{array} \right. \]
+\end{verbatim}
+% 
+Speaking of \pkg{\AmS-math}, they use an entirely different system
+of equation numbering, and this package uses ordinary \LaTeX\ %
+numbering.
+
+\fancybreak
+
+\noindent
+A simple example is:
+%
+\begin{verbatim}
+\begin{numcases}{|x|=}
+  x, & for $x \geq 0$\\
+  -x, & for $x < 0$
+\end{numcases}
+\end{verbatim}
+%
+Giving:
+%
+\begin{numcases}{|x|=}
+  x, & for $x \geq 0$\\
+  -x, & for $x < 0$
+\end{numcases}
+
+\fancybreak
+
+\noindent
+Another example is calculating the square root of $c+id$. First compute
+\phantomsection
+\begin{subnumcases}{\label{w} w\equiv}
+  0    & $c = d = 0$\label{wzero}\\
+  \sqrt{|c|}\,\sqrt{\frac{1 + \sqrt{1+(d/c)^2}}{2}}   & $|c| \geq |d|$ \\
+  \sqrt{|d|}\,\sqrt{\frac{|c/d| + \sqrt{1+(c/d)^2}}{2}}   & $|c| < |d|$
+\end{subnumcases}
+Then, using $w$ from eq.~(\ref{w}), the square root is
+\begin{subnumcases}{\sqrt{c+id}=}
+  0                    & $w=0$ (case \ref{wzero})\\
+  w+i\frac{d}{2w}      & $w \neq 0$, $c \geq 0$ \\
+  \frac{|d|}{2w} + iw  & $w \neq 0$, $c < 0$, $d \geq 0$ \\
+  \frac{|d|}{2w} - iw  & $w \neq 0$, $c < 0$, $d < 0$ 
+\end{subnumcases}
+
+\pagebreak[2]
+
+\noindent
+This was produced by:
+%
+\small
+\begin{verbatim}
+Another example is calculating the square root of $c+id$. First compute
+\begin{subnumcases}{\label{w} w\equiv}
+ 0    & $c = d = 0$\label{wzero}\\
+\sqrt{|c|}\,\sqrt{\frac{1 + \sqrt{1+(d/c)^2}}{2}}   & $|c| \geq |d|$ \\
+\sqrt{|d|}\,\sqrt{\frac{|c/d| + \sqrt{1+(c/d)^2}}{2}}   & $|c| < |d|$
+\end{subnumcases}
+Then, using $w$ from eq.~(\ref{w}), the square root is
+\begin{subnumcases}{\sqrt{c+id}=}
+0                    & $w=0$ (case \ref{wzero})\\
+w+i\frac{d}{2w}      & $w \neq 0$, $c \geq 0$ \\
+\frac{|d|}{2w} + iw  & $w \neq 0$, $c < 0$, $d \geq 0$ \\
+\frac{|d|}{2w} - iw  & $w \neq 0$, $c < 0$, $d < 0$ 
+\end{subnumcases}
+\end{verbatim}
+
+\end{document}