%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-21 on page II-142 in "Display environments as mini-pages".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}
\usepackage{mathtools}
\newcounter{xgnum}
\newcommand\xgnum{\stepcounter{xgnum}%
                       \textbf{\arabic{xgnum}*}}
\newgathered{xgathered}
  {\xgnum\qquad\hfil}{\hfil}{\setcounter{xgnum}{0}}

%StartShownPreambleCommands
% xgathered as defined above
%StopShownPreambleCommands

\begin{document}
\begin{gather}
  \begin{xgathered} x_0=1,\quad x_1=2 \\
                    x_3=3         \end{xgathered}
\end{gather}
\end{document}