The Event_Dispatcher acts as a notification dispatch table. It is used
to notify other objects of interesting things. This information is
encapsulated in Event_Notification objects. Client objects register
themselves with the Event_Dispatcher as observers of specific
notifications posted by other objects. When an event occurs, an object
posts an appropriate notification to the Event_Dispatcher. The
Event_Dispatcher dispatches a message to each registered observer,
passing the notification as the sole argument.