%%
%% The LaTeX Companion, 3ed
%%
%% Example A-2-1 on page II-648 in "Defining and changing counters".
%%
%% 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{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}
%StartShownPreambleCommands
\newcounter{ex} \renewcommand\theex{\thesection.\alph{ex}}
\newenvironment{EX}{\begin{flushleft}%
    \textbf{\refstepcounter{ex}Exercise~\theex:}}
   {\end{flushleft}}
\setcounter{section}{4} % for testing
%StopShownPreambleCommands

\begin{document}
\section{A Failure}
\begin{EX} \label{A} A test.       \end{EX}
\begin{EX} \label{B} Another test. \end{EX}
Referencing exercises: \ref{A} and \ref{B}.
\end{document}