%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-12 on page II-186 in "abraces --- Customizable over and under braces".
%%
%% 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{111.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{0.4\abovedisplayskip}%
    \setlength\belowdisplayskip{0.4\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{abraces,amsmath,color}
\newbracespec{s}{l1D2@{\,\ldots}0@{\color{blue}\ldots\,}2D1r}
%StopShownPreambleCommands

\begin{document}
\[ \aunderbrace[s]{x_1,x_2,x_3,x_4,\dots,x_n} \]
\end{document}