%%
%% The LaTeX Companion, 3ed
%%
%% Example 3-2-5 on page I-151 in "extdash and amsmath --- Dashes in text".
%%
%% 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}
\usepackage{microtype}
%StartShownPreambleCommands
\usepackage[shortcuts,wordspacearound,shortemdash,
            allowbreakbefore]{extdash}
%StopShownPreambleCommands

\begin{document}
\TeX{}'s---basic --- em-dash does not manipulate
surrounding spaces.

Here are\---Gedankenstriche \--- shorter and set
apart. Breaks can\--- as shown \---happen before them.
\end{document}

.
