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|  | Invoking the window-based debugger | 
Whether the text-based or window-based debugger is used is controlled 
using the predicates guitracer/0 
and noguitracer/0. 
Entering debug mode is controlled using the normal predicates for this: trace/0 
and
spy/1. 
In addition, PceEmacs prolog mode provides the command
Prolog/Break at (Control-c b) to insert a 
break-point at a specific location in the source code.
The graphical tracer is particularly useful for debugging threads. 
The tracer must be loaded from the main thread before it 
can be used from a background thread.
Control-c b).guitracer,trace.guitracer,debug.guitracer,spy(Predicate).