Notification on all alerts not matching any notification rule

Notifications can be configured via contacts to pick up specific hosts or services which will generate a notification, with various types of methods and time periods.

AFAIK there is no current mechanism which provides a 'alert for all except those processed by another notification rule' which would be immensely useful to pick up any alerts which do not already have contacts (and therefore specific notifications) defined.

It is obviously possible to pick up all alerts and generate some sort of notification - however that results in duplicate notifications (because the contact based notification generates one notification and the all alerts notification another). The part that is missing is the 'except those processed by another notification rule'.

NB the 'Suppress all previous' does not seem to provide this functionality - in fact its nigh on impossible to identify any effect when this value is set.