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\def\Backslash{\char'134}

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These are the specials supported by {\em dviimp}, as near as I can tell.  In all cases,
distances and diameters are meassured in {\em pt}, and angles are measured in degrees.
\begin{description}
\item[{\tt \Backslash special\{point $n$\}}]\mbox{}\\
This associates the current position on the page
(established by \verb+\put+ or other positioning commands) with the point $n$.  These
points are used by the special \verb+join+ to draw lines.

\item[{\tt \Backslash special\{join  $p$ $n_1\mbox{ }\ldots\mbox{ }n_k$\}}]\mbox{}\\
This connects a sequence
of points established by the \verb+point+ special.  The pen has diameter $p$.  The points
$n_1$ through $n_k$ are connected by $k-1$ lines.

\item[{\tt \Backslash special\{circle $p$ $r$ $\theta_1$ $\theta_2$\}}]\mbox{}\\
This draws a portion of a
circle centered at the current position on the stage.  The pen has diameter $p$, and the
circle has radius $r$.  The arc is drawn counter-clockwise from $\theta_1$ degrees to
$\theta_2$ degrees.  Negative angles are handled, but it is unknown how angles outside
the range -360 to 360 degrees are treated.

\item[{\tt \Backslash special\{ellipse $p$ $r_1$ $r_2$ $\alpha$ $\theta_1$ $\theta_2$\}}]\mbox{}\\
The {\tt ellipse} special draws a portion of an ellipse.  The pen diameter
is $p$.  The ellipse has radius $r_1$
aligned at $\alpha$ degrees, and radius $r_2$ aligned at $\alpha+90$ degrees.
The arc drawn counter-clockwise starts at $\alpha+\theta_1$ and ends at $\alpha+\theta_2$. 
\end{description}
\end{document}