/usr/local/lib/swipl/library/debug.pl
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 debugging(+Topic) is semidet
debugging(-Topic) is nondet
 debugging(?Topic, ?Bool) is nondet
Examine debug topics. The form debugging(+Topic) may be used to perform more complex debugging tasks. A typical usage skeleton is: 
      (   debugging(mytopic)
      ->  <perform debugging actions>
      ;   true
      ),
      ...

The other two calls are intended to examine existing and enabled debugging tokens and are typically not used in user programs.