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<H2><A NAME="sec:12.2"><SPAN class="sec-nr">12.2</SPAN> <SPAN class="sec-title">Tracing 
methods</SPAN></A></H2>

<P>It is often useful to inspect the execution of a particular method. 
Suppose we want to monitor geometry-changes of graphical objects. The 
utility predicate <A NAME="idx:tracepce1:578"></A><A class="pred" href="sec-D.2.html#tracepce/1">tracepce/1</A> 
(see also <A class="sec" href="interface.html">section D</A>) may be 
used:

<PRE class="code">
1 ?- tracepce((graphical-&gt;geometry)).
Trace method: graphical -&gt;geometry
</PRE>

<P>As <CODE>-&gt;</CODE> is a standard Prolog operator with priority 
over 1000,
<A NAME="idx:tracepce1:579"></A><A class="pred" href="sec-D.2.html#tracepce/1">tracepce/1</A> 
requires two brackets. Get-methods may be specified in a similar way. <CODE>&lt;-</CODE> 
is not an operator. It is suggested to define <CODE>&lt;-</CODE> as an 
infix operator in the <font size=-1>XPCE</font> initialisation file, so 
this operator is available during program development:

<PRE class="code">
:- op(100, xfx, &lt;-).
</PRE>

<P>Instance variables may be specified as &lt;<VAR>class</VAR>&gt;-&lt;<VAR>variable</VAR>&gt;. 
A trace-point on an instance variable makes both reading and writing the 
variable visible.

<P>The predicate <A NAME="idx:notracepce1:580"></A><A class="pred" href="sec-D.2.html#notracepce/1">notracepce/1</A> 
disables a tracepoint.

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