Chapter 5: Realizing a Metadata Framework 147 several video clips, audio tracks, and still images for creating a video presentation. The “is used by” relationship is used as a reference count, meaning that it shows how many times an object is used by other objects. Then, while editing the video, you may decide that some clip is not needed, and consequently remove it. In such a case, the clip is not ultimately deleted, but only the relationship with the presentation is removed. Similarly, if you try and delete an included clip using some other application, such as a fi le browser, the system warns that deleting the clip will have adverse consequences for the video presentation. Smart deletion marks the video clip to be deleted later, but does not delete the actual content until its reference count becomes zero (that is, it is no longer used by any other content object, including any application). In Eddie’s loot example, all items Eddie has sent are related to each other by a common character – they are received from Eddie. Since Eddie sends lots of stuff, gigabytes of MP3s, and video clips, it would mean that all items would have a relationship with all other items in the loot. If there were only three or four MP3 fi les, it would be fi ne. It would mean that there are six (three items having two relationships each) or twelve (four items having three relationships) associations between all of them, which would still be easily managed. However, if the number of items increases, the number of relationships increases exponentially, and fi nally the whole system will be drowned by all the relationships. Therefore, we need to have a method to limit the increase of the relationships. We conclude that all relationships are two-way in nature, implying that if the video clip “jumping cat.avi” is used by the movie “all cats. avi”, it automatically means that movie “all cats.avi” uses “jumping cat. avi”. This already cuts the number of relationships in half, since we need to store the relationship only once per pair. Another, better way, to limit relationships is to use those virtual objects that are pure meta- data. Our framework does not enforce it but it is much more effi cient to create a virtual object, here “Eddie’s loot” that has one relationship between all items in the loot. Now the number of relationships increases linearly and what is even better is that Eddie’s loot can have its own metadata. 5.3.4 Usage History and Events An event occurs when a content object is used in any way. It can be a modifi cation event, such as the creation of a fi le, or a non- modifi cation event, such as playing back a video clip. Some events can