%%
%% The LaTeX Companion, 3ed
%%
%% Example A-2-5 on page II-652 in "Defining and changing space parameters".
%%
%% 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{183.0pt}
%StartShownPreambleCommands
\newlength\Mylen
%StopShownPreambleCommands

\begin{document}
\setlength  \Mylen{10mm}  Mylen = \the\Mylen
\addtolength\Mylen{0pt plus 4pt minus 2pt}
\par Mylen = \the\Mylen
\addtolength\Mylen{-10mm plus 1fil}
\par Mylen = \the\Mylen
\end{document}