The interface between the user of a computer-based information system and the system itself has been evolving at a rapid rate. The use of a video screen, with its color and graphics capabilities, has been one factor in this evolution. The development of light pens, mice, and other screen image manipulation devices has been another. With these capabilities has come a natural desire to find more effective ways to make use of them. In particular, much work has gone into the development of interface systems that add visual elements such as icons and graphics to text. The desire to use these...