Tree Doctor : a software package for graphical manipulation and animation of tree structures

User interfaces are increasingly relying on graphical facilities. Since the introduction of the Apple Macintosh, dragging icons seems to have become a popular method for user interfaces on personal computers. In the so-called direct manipulation method (Shneiderman, 1982), the graphical screen and a pointing device (e.g., a mouse, digitizer, or lightpen) are used to simulate the manipulation of real objects. This hands-on approach enables actions like grabbing, dragging, deforming, etc. In computer-aided design technology, where models of real objects are handled, this technique is common practice. But also when the objects are of an abstract kind and exist only within a formal theory, graphical representations may be available which permit the user to treat them in similar ways. In systems like the Macintosh operating system, the potential of the direct manipulation approach is realized to a very limited extent. For instance, files are organized in a well-defined structure (a hierarchical system of directories), which might be graphically represented as a tree. However, users are only allowed to act directly upon files, represented by icons. They cannot handle the stucture itself and are forced to edit it through possibly long sequences of selecting menu items and isolated objects on the screen.