Open fullscreen systems

A general scheme is given for designing APL systems which are both open and fullscreen oriented. Such systems guide the user by presenting her visually the data she is working with, but do not restrict her from using APL to accomplish a task. The system is extensible and modeless in the sense that any commands can be entered at any time during the work with the system. Syntactic abbreviations yield a command language which is easy to learn for users not familiar with APL, but is just APL for those who know it. Finally, some design considerations and requirements are given for screen driver tools which can be used to implement such systems.