public interface BusEvent<T>
+-----------+
| CLEAN | <------------ MAGIC_CLEAN
+-----------+
+-----------+
| DIRTY | <------------ SMUDGE
+-----------+
no state change <------------ CLEAN_ORDER_ERROR
+----------------------+
| CLEANING_ORDERED | <--- CLEAN_ORDER_OK
+----------------------+
+-------------------+
| BEING_CLEANED | <------ CLEAN_STARTED
+-------------------+
+-------------+
| CLEANED | <------------ CLEAN_OK
+-------------+
+-----------+
| FAIL | <-------------- CLEAN_ERROR
+-----------+
+--------+
| * | <----------------- STATE_CHANGE_HACK
+--------+