class property of a thread set by thread_create/3 or set_thread/2.
This implies loading the graphical tracer and switching the thread
to debug mode using debug/0. New threads created inherit their debug
mode from the thread that created them.
Thread classes have been introduced in SWI-Prolog 10.0.2/10.1.5.
This allows for more selective debugging as well as ensuring
debugging works in newly created threads. For example, the HTTP
server creates all its worker threads in the class http. Using
query below, we reliable make sure spy points are trapped in HTTP
handler threads, regardless of whether the worker existed or is
lazily created and regardless of whether the user switched to
nodebug mode while tracing a previous event (see
debug_reset_from_class/0).
?- tdebug(http).