adda/type/exceptions/events declared in the interface:
. the singleton pattern in the cocoa text has an example
where some device mgt needs to know when the app is closing
(ie, the app class provides a notification service,
and it needs to be registered with this service).
if there is an event that type mgt needs,
declaring it in its interface should register it
to receive notifcations of any such events .
. notifications are interrupting the control flow
but unlike during ada exceptions,
the flow can return back to its routine .