INTRODUCTION Although hypermedia is often thought of as something innovative, it has been developed to make explicit already existing, but implicit, relations among pieces of information. A hypermedia model can be used to describe interactive aspects of familiar communication media. A television news program, for example, can be described in terms of a hypermedia presentation---initially there is an introduction by a newscaster; this leads into a film clip, normally accompanied by some commentary, on a particular news story; then we see the newscaster again. In this case the user is not making any choice, but the action of "jumping" to a new scene is present. By extending this example only slightly, a hypermedia presentation can be made by playing the same introduction, then giving the user the choice of which film clips to see, then returning to the newscaster for a further selection. A more static example of interaction is a book, or paper, where a reader can take notes. Se