%%
%% 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}
