Example: DAML

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
            xmlns:daml="http://www.daml.org/2001/03/daml+oil#">
  <daml:Class rdf:about="#Animal">
    <rdfs:comment>
      Animals have exactly two parents, ie:
      If x is an animal, then it has exactly 2 parents 
      (but it is NOT the case that anything that has 2 parents is an animal).
    </rdfs:comment>
    <rdfs:subClassOf>
      <daml:Restriction daml:cardinality="2">
        <daml:onProperty rdf:resource="#hasParent"/>
      </daml:Restriction>
    </rdfs:subClassOf>
  </daml:Class>
</rdf:RDF>