exceptions - events declared in the interface

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).
. generally,
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 .