The current XSD-Type production rule is not declared anywhere. It should be declared as an ID subclass, since anything expanding to an IRI can be a valid datatype.
Not all property values datatypes have to be in the xsd namespace.
To prevent expansion of these datatypes IDs into erroneous IRIs (i.e. most ontologies don't declare an xsd IDspace, so xsd:string would end up being expanded as http://purl.obolibrary.org/obo/xsd_string) the default OWL prefix should also be present as implicit IDspaces in all OBO documents (like BFO and RO already are).
XSD-Typeproduction rule is not declared anywhere. It should be declared as an ID subclass, since anything expanding to an IRI can be a valid datatype.xsdnamespace.xsdIDspace, soxsd:stringwould end up being expanded ashttp://purl.obolibrary.org/obo/xsd_string) the default OWL prefix should also be present as implicit IDspaces in all OBO documents (likeBFOandROalready are).