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error(Formal, Context)exceptions that unify. The tracer is started when a matching exception is raised. This predicate enables debug mode using debug/0 to get more context about the exception. Even with debug mode disabled exceptions are still trapped and thus one may call nodebug/0 to run in normal mode after installing a trap. Exceptions are trapped in any thread. Debug mode is only enabled in the calling thread. To enable debug mode in all threads use tdebug/0.
In many cases debugging an exception that is caught is as simple as below (assuming run/0 starts your program).
?- gtrap(_). ?- run.