%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-4 on page II-182 in "Ellipsis and other kinds of \protect \ldots  ".
%%
%% 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{99.0pt}

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\begin{align*} a &= b+c    &  a &= b+c      \\ \MTFlushSpaceAbove
         & \vdotswithin{=} &  &\vdotswithin{=} \MTFlushSpaceBelow
               x &= y+z    &  x &= y+z    \end{align*}
\end{document}