def set_next(self, frame, step_ignore=0, step_events=None):
        "Sets to stop on the next event that happens in frame 'frame'."
        self.step_events      = None  # Consider all events
        self.stop_level       = Mstack.count_frames(frame)
        self.last_level       = self.stop_level
        self.last_frame       = frame
        self.stop_on_finish   = False
        self.step_ignore      = step_ignore
        return
Sets to stop on the next event that happens in frame 'frame'.