%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 4-8-3 on page 52.
%% 
%%  Copyright (C) 2009 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.
%% 

\documentclass[]{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{194.59192pt}
\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}