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[protege-owl] [Bulk] Re: [empty enumerated class]

Joshua TAYLOR joshuaaaron at
Thu Feb 9 08:09:45 PST 2012

On Wed, Feb 8, 2012 at 12:24 PM, Maatary Daniel OKOUYA
<okouya_d at> wrote:
> Well I try to check the ontology ut i can't even classify it with Pellet, so
> doesn't sound good. About the tab, well i don't understand the link.

You can create empty classes using OWL enumerations, as in the
attached ontology.  I created that, however, by:

1) creating an individual 'b' in the ontology,
2) asserting that the class in question was equivalent to the class { b }
3) saving the ontology and then editing the OWL file in a text editor
to remove b from the list of enumerated individuals

When I opened the resulting file in Protege again, it correctly said
that the class was equivalent to { } (and when I saved again from
Protege, the RDF/XML serialization even used the rdf:nil notation for
the list of enumerated individuals:

    <owl:Class rdf:about="">
                <owl:oneOf rdf:resource="&rdf;nil"/>

As you observed, though, Protege doesn't seem to accept {} as input in
the class expression editor, so I still don't know a way to create
empty enumerations in Protege.

> In any case, my assumption is that the class must be equivalent to the class
> OWL:Nothing. Indeed, i have tried to enter {} in protege 4.1 to mentioned
> that the enumeration is empty but it doesn't work. In any case, my guess is
> that, a class that has its enumeration empty is equivalent to an
> unsatisfiable class.

You're right, an empty class is equivalent to owl:Nothing.  Pellet
will also indicate that such a class is not satisfiable (i.e., that no
individuals can be a member of it).  That's typically a bug in
ontology creation (usually we expect our classes to at least possibly
have members), so Protege/Pellet/otherReasoners might not be all that
happy about it.


Joshua Taylor,
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