Importing accents and wide accents

Question

I know how to import a single symbol from a different font (Importing single symbol from MnSymbol), but it is possible to import an accent?

For example, I would like import \underbrace and \widetilde from \usepackage{mathabx}

Answer 1

I copied the relevant definitions from mathabx.sty and mathabx.dcl.

\documentclass{article}
\makeatletter
\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{
      <5> <6> <7> <8> <9> <10>
      <10.95> <12> <14.4> <17.28> <20.74> <24.88>
      mathx10
      }{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}

%%%%%%%%%
% Tilde %
%%%%%%%%%
\DeclareMathAccent{\widetilde}{0}{mathx}{"72}

%%%%%%%%%%
% Braces %
%%%%%%%%%%
\DeclareMathSymbol{\braceld}{0}{mathx}{"68}
\DeclareMathSymbol{\bracemd}{0}{mathx}{"69}
\DeclareMathSymbol{\bracerd}{0}{mathx}{"6A}
\DeclareMathSymbol{\bracexd}{0}{mathx}{"6B}
\DeclareMathSymbol{\bracelu}{0}{mathx}{"6C}
\DeclareMathSymbol{\bracemu}{0}{mathx}{"6D}
\DeclareMathSymbol{\braceru}{0}{mathx}{"6E}
\DeclareMathSymbol{\bracexu}{0}{mathx}{"6F}

\def\braceexwd{2mu}
\def\bracevkern{0pt}

\def\bracefill#1#2#3#4{$\m@th
    #1#4\mkern-\braceexwd \cleaders\hbox{$#4$}\hfill
    \mkern-\braceexwd#4#2#4\mkern-\braceexwd
    \cleaders\hbox{$#4$}\hfill \mkern-\braceexwd#4#3$}

\def\overbrace@#1#2{\mathop{\vbox{\m@th\ialign{##\crcr
    \noalign{\kern\bracevkern}
    #1\crcr\noalign{\kern\bracevkern\nointerlineskip}
    $\hfil\displaystyle{#2}\hfil$\crcr}}}\limits}

\def\underbrace@#1#2{\mathop{\vtop{\m@th\ialign{##\crcr
    $\hfil\displaystyle{#2}\hfil$\crcr
    \noalign{\kern\bracevkern\nointerlineskip}
    #1\crcr\noalign{\kern\bracevkern}}}}\limits}

\def\overbrace{\overbrace@{\bracefill\braceld\bracemd\bracerd\bracexd}}
\def\underbrace{\underbrace@{\bracefill\bracelu\bracemu\braceru\bracexu}}
\makeatother
\begin{document}
$\underbrace{\widetilde{a+b+c}}$
\end{document}