Underbrace multiple math lines

Question

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

Answer 1

\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}