  In ontology development, there is a gap between domain ontologies which
mostly use the web ontology language, OWL, and foundational ontologies written
in first-order logic, FOL. To bridge this gap, we present Gavel, a tool that
supports the development of heterogeneous 'FOWL' ontologies that extend OWL
with FOL annotations, and is able to reason over the combined set of axioms.
Since FOL annotations are stored in OWL annotations, FOWL ontologies remain
compatible with the existing OWL infrastructure. We show that for the OWL
domain ontology OBI, the stronger integration with its FOL top-level ontology
BFO via our approach enables us to detect several inconsistencies. Furthermore,
existing OWL ontologies can benefit from FOL annotations. We illustrate this
with FOWL ontologies containing mereotopological axioms that enable new
meaningful inferences. Finally, we show that even for large domain ontologies
such as ChEBI, automatic reasoning with FOL annotations can be used to detect
previously unnoticed errors in the classification.
