%% 
%%  Example 01-11-1 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{86.31393pt}

%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\begin{align*}
a & \rightarrow b\\
  & \hphantom{\rightarrow} b\\
  & \mkern\thickmuskip\hphantom{\rightarrow}\mkern\thickmuskip b\\
  & \mathrel{\hphantom{\rightarrow}} b
\end{align*}
\end{document}
