Barbara,

> here's what it says in the file technote.tex that's distributed with the
> ams-latex collection.
> ...
> Users have proposed two workarounds (September 1996):
> ....

Both of those workarounds always looked odd to me, as as far as I can
see the code goes to some lengths to make the following construction
work, but then doesn't tell anyone about it. The same thing works for
essentially the same reasons in the verbatim and tabularx package
environments, those package document this though.

David

\documentclass[]{article}

\usepackage[]{amsmath}

\newenvironment{fancyequation}
{\hrule \equation}
{\endequation\hrule}

\begin{document}

\begin{fancyequation}
a = b
\end{fancyequation}

\end{document}