Removes a client from the set of registered clients. Calls the client's unregistered() method, then calls the abstract remove_() method.
Adds a client to the set of registered clients. Calls the client's registered() method, then calls the abstract add_() method.
Base class for a set of registered clients, with methods to add & remove clients, and to tell how many clients are currently registered. When a client is added or removed from the set its registered() or unergistered() method, respectively, is called automatically.