It is also important to note that while both topic associations and normal cross references are hyperlinks, they are very different creatures: In a cross reference, the anchors (or end points) of the hyperlink occur within the information resources (although the link itself might be outside them); with topic associations, we are talking about links (between topics) that are completely independent of whatever information resources may or may not exist or be considered as occurrences of those topics. Why is this important? Because it means that topic maps are information assets in their own right, irrespective of whether they are actually connected to any information resources or not. The knowledge that Rome is in Italy, that Tosca was written by Puccini and is set in Rome, etc. etc. is useful and valuable, whether or not we have information resources that actually pertain to any of these topics. Also, because of the separation between the information resources and the topic map, the same topic map can be overlaid on different pools of information, just as different topic maps can be overlaid on the same pool of information to provide different “views” to different users. Furthermore, this separation provides the potential to be able to interchange topic maps among publishers and to merge one or more topic maps.[6]