%% typed May 30, 2000
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\begin{document}
\section*{Equation With Left And Right Tags}
As a special effect, you may occasionally want a displayed equation 
to have a tag both on the left and on the right of the equation line,
as in the two following examples:
$$
\Longrightarrow \setbox0=\hbox{$\Longrightarrow$}\kern-\wd0
  \hspace{.5\columnwidth minus .5\columnwidth}
       g(x)=\int_a^b dx\, \frac{f(x)}{\sqrt{1-x^2}}
       \hspace{.5\columnwidth minus .5\columnwidth}
       \llap{\em a difficult integral}
$$
$$
\Longrightarrow 
  \hspace{.5\columnwidth minus .5\columnwidth}
       g(x)=\int_a^b dx\, \frac{f(x)}{\sqrt{1-x^2}}
       \hspace{.5\columnwidth minus .5\columnwidth}
       \mbox{\em a difficult integral}
$$
\bigskip

\noindent
The first example, with the equation centered on the line, is product by
\begin{verbatim}
$$
\Longrightarrow \setbox0=\hbox{$\Longrightarrow$}\kern-\wd0
  \hspace{.5\columnwidth minus .5\columnwidth}
       g(x)=\int_a^b dx\, \frac{f(x)}{\sqrt{1-x^2}}
       \hspace{.5\columnwidth minus .5\columnwidth}
       \llap{\em a difficult integral}
$$
\end{verbatim}

\bigskip
\noindent
The second example, with the equation centered between the tags, is product by
\begin{verbatim}
$$
\Longrightarrow 
  \hspace{.5\columnwidth minus .5\columnwidth}
       g(x)=\int_a^b dx\, \frac{f(x)}{\sqrt{1-x^2}}
       \hspace{.5\columnwidth minus .5\columnwidth}
       \mbox{\em a difficult integral}
$$
\end{verbatim}
\end{document}