document: Just the right amount : extracting modules from ontologies The ability to extract meaningful fragments from an ontology is key for ontology re-use . We propose a definition of a module that guarantees to completely capture the meaning of a given set of terms , i.e. , to include all axioms relevant to the meaning of these terms , and study the problem of extracting minimal modules . We show that the problem of determining whether a subset of an ontology is a module for a given vocabulary is undecidable even for rather restricted sub-languages of OWL DL . Hence we propose two `` approximations '' , i.e. , alternative definitions of modules for a vocabulary that still provide the above guarantee , but that are possibly too strict , and that may thus result in larger modules : the first approximation is semantic and can be computed using existing DL reasoners ; the second is syntactic , and can be computed in polynomial time . Finally , we report on an empirical evaluation of our syntactic approximation which demonstrates that the modules we extract are surprisingly small .
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