%% 
%%  Example 01-14-4 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{158.31393pt}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
\makeatletter
\def\@bracketheight{2pt}
\def\bracketend#1#2{\vrule height #2 width #1\relax}
\def\overbracket{\@ifnextchar[{\@overbracket}{\@overbracket[\@bracketheight]}}
\def\@overbracket[#1]{\@ifnextchar[{\@over@bracket[#1]}{\@over@bracket[#1][0.3em]}}
\def\@over@bracket[#1][#2]#3{%\message {Overbracket: #1,#2,#3}
   \mathop {\vbox {\m@th \ialign {##\crcr \noalign {\kern 3\p@
   \nointerlineskip }\downbracketfill {#1}{#2}
        \crcr \noalign {\kern 3\p@ }
        \crcr  $\hfil \displaystyle {#3}\hfil $%
        \crcr} }}\limits}
\def\downbracketfill#1#2{$\m@th \setbox \z@ \hbox {$\braceld$}
    \edef\@bracketheight{\the\ht\z@}\downbracketend{#1}{#2}
    \leaders \vrule \@height #1 \@depth \z@ \hfill
    \leaders \vrule \@height #1 \@depth \z@ \hfill
\downbracketend{#1}{#2}$}
\def\downbracketend#1#2{\vrule depth #2 width #1\relax}
\makeatother
%StartShownPreambleCommands
% has the underbracket stuff in hidden part
%StopShownPreambleCommands
\begin{document}
\begin{tabular}{cl}
1.& $\overbracket {foo\ bar}$ \\
2.& $\overbracket[2pt]{foo\ bar}$\\
3.& $\overbracket[2pt][1em]{foo\ bar}$
\end{tabular}
\end{document}
