Two column align environment with one line spanning both columns

Is there a good/correct way of making an align environment with one line spanning multiple columns?

At the moment I'm using a hack like the following, but I feel that it's probably not the best way.

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
L1C1 &= 1234 \,,&     L1C2 &= abcd \,,\\
L2C1 &= 1234 \,,&     L2C2 &= efgh \,,\\
L3 &= 1234567890abcdefghijklmnop \ . \hspace{-30cm}
\end{align}
\end{document}


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another hack is to \rlap the part of the wider lines that would exceed the width of the first column. in the example given, i would, for the last line, put L3 &= 1234567890\rlap{$abcdefghijklmnop \ .$}. most lines of math contain reasonable places where one can break without damage. – barbara beeton Apr 15 '11 at 12:17
@barbara: You should post that as an answer. – Loop Space Apr 15 '11 at 17:35

this is also a hack, but it will give the desired result.

use \rlap to "hide" the width of the overlong line. for the example given, i would do this for the last line:

L3 &= 1234567890\rlap{$abcdefghijklmnop \ .$}


you might even want to put some extra space after that to get the desired separation; just look at the output to see what's needed.

most lines of math contain reasonable places where one can break without damage.

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with package mathtools one can write \mathrlap{abcd...} – Herbert Apr 15 '11 at 17:54
I always forget the *lap commands (but for some reason smash stays in my head). @Herbert Thanks for the mathrlap command from mathtools. I think that's the cleanest solution given yet. – Simon Apr 15 '11 at 23:28

To span you could use \span:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
L1C1 &= 1234 \,,&     L1C2 &= abcd \,,\\
L2C1 &= 1234 \,,&     L2C2 &= efgh \,,\\
L3 &= {}\span\omit$1234567890abcdefghijklmnop$\ .\hidewidth
\end{align}
\end{document}


It turned out to be a lot more hacky than I initially thought it would be.

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\span does not have the proper "class" for tex to be able to determine the spacing (see table, texbook, p.170). if you precede \span by {} that would obviate the need for \;. – barbara beeton Apr 15 '11 at 20:09
This changes the position of the equation numbering. Here's a comparison. – Simon Apr 15 '11 at 23:29
@barbara: thanks for the info. – morbusg Apr 16 '11 at 5:26
@Simon: Edited. – morbusg Apr 16 '11 at 5:34
Thanks morbusg! – Simon Apr 16 '11 at 7:30