Equations moving in overlays

Dear all,

I am having trouble with the following:

%---------------------------------------------------------------------- SLIDE -
\overlays{2}{%
\begin{slide}{An equation}
\begin{displaymath}
\fromSlide{1}{L=ab}%
\fromSlide{2}{+c}%
\end{displaymath}
\end{slide}
}
%------------------------------------------------------------------------------

I would like the output of the equation to be incremental (which it is) but when I forward between overlays the position of the first part of the equation moves slightly. How do I prevent this?

Thanks,
Gordon McGregor.



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