def event(self, event_name):
        """Register an event that occurred during processing of a task of the given
        type.

        Args:     event_name: str         A name for a type of events. Events of the
        same type are displayed as         a single entry and a total count of
        occurences.

        """
        self._event_dict.setdefault(event_name, 0)
        self._event_dict[event_name] += 1
        self._log_progress_if_interval_elapsed()