%% 
%%  An UIT Edition example
%% 
%%  Example 08-01-12 on page 154.
%% 
%%  Copyright (C) 2010 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{375.57637pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\newcommand*\diff{\mathop{}\!\mathrm{d}}
\newlength\PREsave
\StopShownPreambleCommands
\begin{document}
The effective \ldots
\[
  \fbox{\the\predisplaysize}\quad F(x)=\int f(x)\diff x
  \global\PREsave=\predisplaysize
\]
\advance\PREsave by -2em\rule{\PREsave}{1pt}
\end{document}