WSML Deliverable D 20 . 1 v 0 . 2 OWL − WSML Working Draft – January 6 , 2005

This deliverable presents restricted variants of the OWL Lite, DL and Full species of the OWL ontology language, called OWL Lite−, OWL DL− and OWL Full−, respectively. OWL Lite− and OWL DL− are strict subsets of OWL Lite and OWL DL. They are limited in such a way that there exists a translation from both languages directly into dunction-free logic programs, i.e., Datalog. Thus, any OWL Lite− or OWL DL− ontology can be translated into Datalog. An ontology language for which a translation to Datalog exists has several advantages. Most notably, it can benefit from long-term research in the field of deductive databases resulting in highly optimized query answering engines, and allows for easy implementation of a rule and a query language on top of the ontology. It turns out that most current ontologies fall inside the fragments defined by OWL Lite− or OWL DL−. We describe the restrictions on the OWL Lite abstract syntax and provide an analysis of the features of OWL Lite, which are not included in OWL Lite−, and give a rationale for not including them in the language. We present the RDF syntax and the model-theoretic semantics for OWL Lite− and relate OWL Lite− with RDFS. It turns out that there are features in OWL DL, which can still be included in the language, while still allowing for efficient query answering. Therefore, we present extensions of OWL Lite− in order to create OWL DL−, together with its abstract syntax and model-theoretic semantics. Then, the in our view most useful feature of OWL Full, the possibility to treat classes as instances, can also be translated to a positive Datalog program. We describe a (Datalog subset of) F-Logic based extension of OWL DL− to create OWL Full−. The overall goal of OWL− is not necessarily to serve as an own ontology language but defining a maximal subset of the original OWL variants which does not fall outside a logic programming framework in order to allow for a straight-forward combination with rule languages. We believe that the OWL− subset of OWL can serve as a starting point for a more usable Web Ontology language and extensions in various directions which tackle some of the limitations of OWL outlined in this deliverable: Extensions of OWL Full− include the addition of Datatypes and Database-like constraints which are discussed in more detail in Deliverable 20.3 of WSMO [de Bruijn et al., 2004]. Copyright c © 2004 DERI R © , All Rights Reserved. DERI liability, trademark, document use, and software licensing rules apply.