Figure : A Protégé screenshot showing the metadata of an ontology class labeled as chemical reaction, annotated with a definition that was derived from the Gold Book definitions of the terms chemical reaction, chemical species, molecular entity, reagent, and product. To indicate this use of the Gold Book as the primary source, the definition itself is annotated on the axiom level using the PROV ontology predicate hadPrimarySource multiple times to reference each of these GoldBook term definitions via their respective DOIs.