Look at the following example

\begin{align}
& x \\
\overset{\text{some long explanation}}= & foo \\
& = bar
\end{align}


what is the best way to get the equal signs aligned? Now I have seen different solution such as adding \overset{\hphantom{\ţext{some long explanation}}}= to the beginning of every line, but I don't really think that is the best solution. Any better ideas?

UPDATE

I guess what i am actually after is

1. aligning the first column of the align environment right
2. aligning the equal sign under the overset text right
-
Please correct your current code snippet, since I assume you're after some horizontal alignment. You just mention "align" but provide a horizontally set align. At the moment it's not entire clear what you're after. –  Werner Jan 7 '12 at 15:50
yes. I guess what you mean where the missing \\s? –  niklasfi Jan 7 '12 at 16:18
If you modify \mathclap in Herbert's answer to \mathllap, you get something more in line with what you're after. If this is still not the case, could you include an ASCII art (or some other drawing/image) of the exact alignment you're after? –  Werner Jan 7 '12 at 17:09

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\begin{align}
x & \\
&\overset{\mathclap{\strut\text{some long explanation}}}= foo\\
& = bar
\end{align}

\end{document}

-
this leads to some long text floating into the formula after the equal sign –  niklasfi Jan 7 '12 at 16:39
I find \strut to be unrelated with the problem, plus it creates some unwanted vertical space. \mathclap alone seems to be sufficient to solve the issue. –  Raphael Oct 10 '13 at 14:40
You only want to ignore the left overlapping part of the some long text for the align environment while properly aligning the following text foo according to standard math-alignment rules. This has not been answered here. –  Jost Oct 17 '13 at 14:06

you say you don't want to repeat "some long explanation", but there really isn't any better way to do it that i know of.

is this what you're after?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
x &\overset{\text{some long explanation}}= foo \\
&\overset{\phantom{\text{some long explanation}}}= bar
\end{align}
\end{document}

-
Thanks, this works for me, but it is not very neat. –  quapka Apr 22 at 12:26