Introduction From the early days of computing, many individuals have recognized that algebras provide interesting mathematical models for at least some aspects of programs. In mathematics, an algebra consists of a set (called the carrier of the algebra), together with a finite set of total functions that have the carrier set as their common codomain. The algebras we learn in school, however, are usually those derived from number theory and programs are more diverse, if not richer, than operations on numbers. A somewhat more abstract notion, called signature algebras, has been used for some time to to model abstract data types [GTW78]. A signature defines a set of typed operator symbols without specifying functions that would be the actual operators. Thus a signature defines a class of algebras, namely the algebras whose operators conform to the typing constraints imposed by the signature. Signature algebras have been helpful in understanding the issues involved in abstract dat