All versions of the UMBEL Ontology & Reference Structure are available on a SVN versioning repository.
You can browse and access the SVN server on this page. All past versions of the ontology are accessible in the trunk of the svn, in their versions folders. All the RDF files describing the ontology and the conceptual structures are archived on the SVN repository.
All the XML and N3 files of the vocabulary and the reference structure in the root folder in the trunk are the latest versions of the vocabulary and the reference structure. Everything in the folders are past version of the ontology.
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