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14. TOOLS: ONTOLOGY SERVERThe ontology server (Farquhar et al., 1995) is a set of tools and services that support the building of shared ontologies between geographically distributed groups. It was developed in the context of the ARPA Knowledge Sharing Effort by the Knowledge Systems Laboratory at Stanford University. This server is an extension of Ontolingua. Initially, the term Ontolingua was used to refer to both the language in which ontologies are expressed formally and the tool used to build such ontologies. Actually, when the community uses this term, it refers to the language provided by the ontology server. Initially, the world was conceptualized in Ontolingua using relations, functions, classes, individuals, and axioms (Gruber, 1993a). Relations are defined as a set of tuples, where each tuple is a list of objects. Functions are a special case of relation, in which the last object in each tuple is unique, given the preceding objects. Classes are unary relations of one argument. Individuals are instances. Basically, all definitions in Ontolingua present the same pattern: an informal definition in natural language and a formal definition using KIF statements. Before each KIF sentence, a keyword appears: :def, to express necessary conditions; :iff-def, to express necessary and sufficient conditions; :lambda-body, to express the value of a function in terms of its arguments; and, :axiom-def, to constrain the object constant. With the aim was to move from a relational representation toward a frame-based or object-oriented representation, the Frame-Ontology was built. Ontolingua allows the use of both languages simultaneously. However, their syntaxes are quite different. For example, the statement "dogs are mammals" in KIF is codified as :def (... (mammal ?dog) ...) and, using the frame ontology, as (subclass-of dog mammal). As stated by Gruber (1993a), Ontolingua is inherently incomplete with respect to the KIF language since not all knowledge codified in KIF can be codified using the frame ontology. To solve this problem, Ontolingua allows the inclusion of KIF expressions in definitions made using the frame ontology. However, the translation programs do not support these statements. Ontolingua is also a domain-independent tool (Gruber, 1993a). The portability of Ontolingua ontologies is in its translation architecture, which translates any ontology into different representation languages (e.g., prolog, loom, Epikit, KL-ONE style system, pure-kif), but not back again. Indeed, Ontolingua was accepted by the knowledge-sharing community as the main tool to implement ontologies due to this set of translators. Ontolingua evolved toward the Ontology Server. The ontology server architecture provides access to a library of ontologies, translators to languages (LOOM, IDL, CLIPS, etc.), and an editor to create and browse ontologies. There are three modes of interaction: remote collaborators that are able to write and inspect ontologies; remote applications that may query and modify ontologies stored at the server over the Internet; and stand-alone applications. Browsing, creating, editing, maintaining, using, and sharing ontologies, as well as collaborative development by different groups are the most representative facilities provided by this Ontology Server. The ontology server may be accessed through the URL: http://www-ksl-svc.stanford.edu:5915/.
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