%% 
%%  An UIT Edition example
%% 
%%  Example 04-08-3 on page 50.
%% 
%%  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{177.70511pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\setcounter{equation}{20}
\renewcommand\theequation{4.\arabic{equation}}
\StartShownPreambleCommands

\StopShownPreambleCommands
\begin{document}
\begin{equation}a \rightarrow b\end{equation}
\begin{equation}
a \mkern\thickmuskip\hphantom{\rightarrow}
 \mkern\thickmuskip b
\end{equation}
\begin{equation}
a \mathrel{\hphantom{\rightarrow}} b
\end{equation}
\end{document}