Underbrace multiple math lines

I have to underbrace multiple lines in an align-environment with one brace. Meaning something like

\begin{align*}
A ={}& B + \underbracemultiline{C + D + E \\
& + F + G \\
& + H + I \\
& + J + K}_{=:M} + L
\end{align*}


results in (a nice version of)
http://www.phoenixes.de/Bsp.png http://www.phoenixes.de/Bsp.png
Another possible syntax would be

\begin{align*}
A ={}& B + \under{brace}{line}{dots}{C + D + E} \\
& \under{dots}{brace}{dots}{+ F + G}_{=:M} \\
& \under{dots}{line}{dots}{+ H + I} \\
& \under{dots}{line}{brace}{+ J + K} + L
\end{align*}


where the macro

\under{begin}{middle}{end}{What to under"line"}


is as needed for the example. How can I do this?

Thx

\underbrace is constructed from bits so you just need to copy its definition using different bits each time:

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\underbracex#1#2{\mathop{\vtop{\m@th\ialign{##\crcr
$\hfil\displaystyle{#2}\hfil$\crcr
\noalign{\kern3\p@\nointerlineskip}%
#1\crcr\noalign{\kern3\p@}}}}\limits}

\def\underbracea{\underbracex\upbracefilla}

\def\upbracefilla{$\m@th \setbox\z@\hbox{$\braceld$}% \bracelu\leaders\vrule \@height\ht\z@ \@depth\z@\hfill \kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@$}

\def\underbraceb{\underbracex\upbracefillb}

\def\upbracefillb{$\m@th \setbox\z@\hbox{$\braceld$}% \vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@ \leaders\vrule \@height\ht\z@ \@depth\z@\hfill\bracerd \braceld\leaders\vrule \@height\ht\z@ \@depth\z@\hfill \kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@$}

\def\underbracec{\underbracex\upbracefillc}

\def\upbracefillc{$\m@th \setbox\z@\hbox{$\braceld$}% \vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@ \leaders\vrule \@height\ht\z@ \@depth\z@\hfill \kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@$}

\def\underbraced{\underbracex\upbracefilld}
\def\upbracefilld{$\m@th \setbox\z@\hbox{$\braceld$}% \vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@\vrule \@width\p@\kern\p@ \leaders\vrule \@height\ht\z@ \@depth\z@\hfill\braceru$}

\makeatother
\begin{document}

\begin{align*}
A ={}& B + \underbracea{C + D + E} \\
&\underbraceb{{} + F + G}_{=:M} \\
&\underbracec{{}+ H + I} \\
&\underbraced{{} + J + K} + L
\end{align*}

\end{document}