Entity-Relationship model (Chen, 1976), and with an object-oriented language in the style of the one introduced by Abiteboul and Kanellakis (1989). More specifically, we identify the most relevant features to model classes in each of the cited settings and show that a specification in any of those class-based formalisms can be equivalently expressed as a knowledge base in ALUNI. In this way, we are able to identify which are the commonalities among the families and which are the specificities of each family. Therefore, even though there are specific features of every family that are not addressed by ALUNI, we are able to show that the formalism proposed in this paper provides important features that are currently missing in each family, although their relevance has often been stressed. In this sense, our unifying framework points out possible developments for the languages belonging to all the three families.

One fundamental reason for regarding ALUNI as a unifying framework for class-based representation formalisms is that reasoning in ALUNI is hard, but nonetheless decidable, as shown by Calvanese, Lenzerini, and Nardi (1994), Calvanese (1996c).  Consequently, the language features arising from different frameworks to build class-based representations are not just given a common semantic account, but are combined in a more expressive setting where one retains the capability of solving reasoning tasks. The combination of constructs included in the language makes it necessary to distinguish between reasoning with respect to finite models, i.e., models with a finite domain, and reasoning with respect to unrestricted models.  Calvanese (1996c) devises suitable techniques for both unrestricted and finite model reasoning, that enable for reasoning in the different contexts arising from assuming a finite domain, as it is often the case in the field of databases, or assuming that a domain can also be infinite. In the paper, we discuss the results on reasoning in ALUNI, and compare them with other results on reasoning in class-based representation formalisms.

Summarizing, our framework provides an adequate expressive power to account for the most significant features of the major families of class-based formalisms. Moreover, it is equipped with suitable techniques for reasoning in both finite and unrestricted models.

Therefore, we believe that ALUNI captures the essential core of the class-based representation formalisms belonging to all three families mentioned above.

The paper is organized as follows. In the next section we present our formalism and in Sections 3, 4, and 5 we discuss three families of class-based formalisms, namely, frame languages, semantic data models, and object-oriented data models, showing that their basic features are captured by knowledge bases in ALUNI. The final sections contain a review of related work, including a discussion of reasoning in ALUNI and class-based formalism, and some concluding remarks.

## 2. A Unifying Class-Based Representation Language

In this section, we present ALUNI, a class-based formalism in the style of description logics (DLs) (Brachman & Levesque, 1984; Schmidt-Schauß & Smolka, 1991; Donini et al., 1996; Donini, Lenzerini, Nardi, & Nutt, 1997). In DLs the domain of interest is modeled by means of concepts and roles, which denote classes and binary relations, respectively.  Generally speaking, a DL is formed by three basic components:
- A description language, which specifies how to construct complex concept and role expressions (also called simply concepts and roles), by starting from a set of atomic