LINK's graphical user interface (GUI) has been written in such a way that
extensions and changes are much easier than would be possible
with an XWindows application.  Erick Gallesio's object-oriented Scheme 
interface (STk)~\cite{eg95} to John Ousterhout's Tk graphics 
package~\cite{Ous94} makes this possible.  It is not necessary to know
any further details of these packages in order to use LINK effectively,
but the LINK User's Manual (see Page~\pageref{part:users-manual} describes 
how to make effective use of some power features of the system.  For
example, it is possible to program series of computations such as groups
of experiments, and systematic evaluation and/or setting of attributes 
by writing very small Scheme programs.

The GUI is intended to be usable without documentation.  However, we
will briefly describe its features below.  In our descriptions, we will
use the following notation to describe events:

\begin{tabular}{|l|l|} \hline
Notation		& 	Expanation	\\ \hline\hline
{\em Main Window} 	&	The small window that comes up when LINK is run \\ \hline
{\em Graph View} 	&	A window in which graphs can be drawn.\\ \hline
{\em Graph Menu} 	&	The menu at the top of a graph window.\\ \hline
{\em Menu-option} 	&	Select {\em option} from {\em Menu} \\
{\em Menu1-Menu2-option}&	Selecte the {\em Menu2} option from \\
			&       {\em Menu1}, then select {\em option} \\
			&       from {\em Menu2}. \\
$\ldots$		&       and so on. \\ \hline
$<1st>$			& 	Click with the first (leftmost) mouse button\\\hline
$<2nd>$			& 	Click with the second (middle) mouse button\\\hline
$<3rd>$			& 	Click with the third (rightmost) mouse button\\\hline
$<Shift-1st>$		& 	Click the first mouse button while holding the shift key down\\\hline
\end{tabular}