%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-10 on page II-185 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{123.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.3\abovedisplayskip}%
    \setlength\belowdisplayskip{.1\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{abraces,amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{gather*}
  \aoverbrace [L1U3R] {a^3 +3ab^2 + 3a^2b +b^3}           \\
  \aunderbrace[l1U1D1U1r] {\text{Some text with a brace}} \\
  \aunderbrace[l10@{\ldots}04r] {x_1,x_2,x_3,\dots,x_n}   \\
  \aunderbrace['1'] {\smash[b]{(a+b)}}\nolimits^2
    = \aunderbrace['1,[5pt]1'] {\smash[b]{\; a^2+2ab+b^2}}
\end{gather*}
\end{document}