How to handle formulas which are to long for a side?

[kulla]

Sometimes we have formulas which are to long to fit inside \textwidth. Currently our authors need to rewrite them. We might look into a solution for automatically resizing them, in case they are too big. There ist for example the package adjustbox. The command

\adjustbox{max width=0.8\textwidth}{<content>}

would resize <content> when it is too wide (it works like \resizebox of graphics). However, ` will be in text mode. We could use

\begin{adjustbox}{max width=0.9\textwidth}
\begin{center}
$\begin{aligned}
.....
\end{aligned}$
\end{center}
\end{adjustbox}

But this wouldn't work together with \allowdisplaybreaks. There is also the package resizegather for automatically resizing the environment gather, but we need a package for the command align*.

[vroland]

May also be considered in #133.