A frustrating aspect of the frequent changes to large VLSI CAD systems is that so little of the old available programs can be reused. It takes too much time and effort to find the reusable pieces and recast them for the new use. Our thesis is that such systems can be designed for reusability by designing the software as layers of problem oriented languages, which are implemented by suitably extending a "base" language. We illustrate this methodology with respect to VLSI CAD programs and a particular language layer: a language for handling networks. We present two different implementations. The first uses UNIX and Enhanced C. The second approach uses Common Lisp on a Lisp machine.
