%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-3-7 on page II-157 in "amsmath, mathtools, cases --- Some case environments".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See
https://www.latex-project.org/lppl.txt for details.
%%
\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}
%StartShownPreambleCommands
\usepackage{amsmath,cases}
%StopShownPreambleCommands
\begin{document}
\begin{subnumcases}{f(x) = \label{L1}}
-1 & if $x<0$\,, \\
0 & if $x=0$\,, \tag{$*$} \\
1 & if $x>0$\,. \label{L2}
\end{subnumcases}
Reference to a subequation \eqref{L2} and to
the overall equation \eqref{L1} are possible.
\end{document}
