Program development and documentation by step-wise transformations: An interctive tool

The process of program development is an evolutionary process whose evolving steps must be properly documented for ease of understanding and evolution. The various steps of the design process are "photographed" by DUAL and the "movie" showing the derivation phases of each piece of program code starting from its preliminary design idea down to its actual implementation - can be dynamically "replayed" on the screen at the visitor's (designer or maintenance engineer) will. A clear distinction between design, implementation and documentation no longer exists in DUAL. The description of a design is but a description of the implementation at higher level of abstraction and is expressed in a design language which, by successive transformations, will evolve into the program text.