Special Issue: Representing functionality in design
, As designs exist to satisfy some purpose or function, knowledge of functionality is essential in a wide variety of designrelated activities, including generation and modification of designs, comparison, evaluation and selection of designs, and explanation, diagnosis or repair of designs. Functional modelling refers to a wide variety of approaches to model a design and its requirements from its functional aspects so as to allow reasoning about its functionality for various activities such as the above. Function has been historically interpreted in a wide variety of ways: for instance, as an abstraction of the intended behavior of a design, an indexing of its intended behavior, the relationship between a design and its environment, the external behavior of a design, or its internal behavior. Functional reasoning as a design approach has been around for at least 25 years now (Koller, Rodenacker, and Roth in Germany, French in the United Kingdom, Freeman and Newell in the United States, Hubka in Switzerland, and Yoshikawa in Japan are but a few examples of the early researchers in this area), and have attempted to support design in the conceptual stage by methods and approaches to describe function, to establish function structures (e.g., by using generally valid functions), to satisfy these subfunctions and combine them into concept alternatives (e.g., by using catalogues of physical effects and working principles), and to evaluate these (e.g., by using morphological matrices). However, the emphasis was largely prescriptive, and computer supports passive in nature. The advent of computers and the development of artificial intelligence (AI) techniques provided a renewed focus on reasoning about functions, extending the area into diagnosis and explanation, and allowed computers to take a more active role in the design process, especially in its generative aspects. A formal representation of functionality is essential for supporting any of th€se activities on computers. Tradition-