%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-13 on page II-187 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{0.1\abovedisplayskip}%
    \setlength\belowdisplayskip{0.1\belowdisplayskip}%
  }
  % Using too many &s in _{...} generates an error message
  % but for the book production that would stop the process (for good reason)
  % so we redefine \PackageError to become a warning only:

  \renewcommand\PackageError[3]{\PackageWarning{#1}{#2.\MessageBreak
      This is normally an error}}


%StartShownPreambleCommands
\usepackage{abraces,amsmath}
\newbracespec{s}{L1U1A1U1R}
%StopShownPreambleCommands

\begin{document}
\begin{gather*}
  \aoverbrace[s]{\text{a braced formula}}^{x \& y}         \\
  \aoverbrace[s]{\text{a braced formula}}^{x & y}          \\
  \aoverbrace[s]{\text{a braced formula}}^{x & & z}        \\
  \aoverbrace[s]{\text{a braced formula}}^{ & & z}
    _{{\begin{smallmatrix}a&b\\c&d\end{smallmatrix}} & dropped}
\end{gather*}
\end{document}