Customers do not want frozen specifications

A technique is described that has been used to introduce flexibility into the specification of an interactive computer system. For the most part, the specification is written in English, but the portion of interest here uses a graphical notation which has a precise meaning, yet can readily be understood by a non-specialist. This portion of the specification can easily be changed in a controlled manner. The technique was carried right through to the design and implementation, which retained the same flexibility. It is described here by means of a simple illustrative example