  Extracting a subset of a given OWL ontology that captures all the ontology's
knowledge about a specified set of terms is a well-understood task. This task
can be based, for instance, on locality-based modules (LBMs). These come in two
flavours, syntactic and semantic, and a syntactic LBM is known to contain the
corresponding semantic LBM. For syntactic LBMs, polynomial extraction
algorithms are known, implemented in the OWL API, and being used. In contrast,
extracting semantic LBMs involves reasoning, which is intractable for OWL 2 DL,
and these algorithms had not been implemented yet for expressive ontology
languages. We present the first implementation of semantic LBMs and report on
experiments that compare them with syntactic LBMs extracted from real-life
ontologies. Our study reveals whether semantic LBMs are worth the additional
extraction effort, compared with syntactic LBMs.
