The ProXL callback mechanism allows the user to specify an exit variable associated with each callback routine. After the successful execution of a callback routine, ProXL examines the exit variable, if it exists.
If the callback succeeded and the exit variable was bound by the callback
to a value that
unifies with the ExitCond given, handle_events/[1,2,3],
and dispatch_event/[2,3] succeed, with ExitCond
bound to the value given by the callback routine.