Nothing in App. B of OWL Language Reference 31 March 2003

Appendix B says

<Class rdf:ID="Thing">
  <rdfs:label>Thing</rdfs:label>
  <unionOf rdf:parseType="Collection">
    <Class rdf:about="#Nothing"/>
    <Class>
      <complementOf rdf:resource="#Nothing"/>
    </Class>
  </unionOf>
</Class>

<Class rdf:ID="Nothing">
  <rdfs:label>Nothing</rdfs:label>
  <complementOf rdf:resource="#Thing"/>
</Class>

Since the 2nd group implies that complementOf Nothing is Thing,
the first group is equivalent to

<Class rdf:ID="Thing">
  <rdfs:label>Thing</rdfs:label>
  <unionOf rdf:parseType="Collection">
    <Class rdf:about="#Nothing"/>
    <Class rdf about="#Thing"/>
  </unionOf>
</Class>

which is probably what you want to say,
or else omit it all together.  

Even better would be to replace "Nothing" with
some variant of "any Class" -- which I believe
is what the document says elsewhere.
============ 
Dick McCullough 
knowledge := man do identify od existent done;
knowledge haspart proposition list;