%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-5-17 on page II-189 in "underoverlap --- Partly overlapping horizontal 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{135.0pt}

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

%StartShownPreambleCommands
\usepackage{underoverlap,amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{gather} a + \UOLoverbrace{b +}[c + d]^{\text{both}}
                   \UOLunderline{+ f}_{\text{fine}} + g  \\
 a + \UOLoverline{b +}[c + d]^{\text{wrong!!}} + f + g   \\
 a + \UOLoverbrace{b +}[c + d]^{\text{clearly wrong}}
   + f \UOLunderbrace{+ g}_{\text{too!}}       \end{gather}
\end{document}
