Relation pm#different__different_from__differentfrom (?,?)
  exclusion:  pm#equal
  supertype:  pm#irreflexive_relation  pm#symmetric_relation  pm#antisymmetric_relation
  subtype:  owl#different_from__differentfrom (?,?)
  subtype:  pm#exclusive_class__exclusiveclas (rdfs#class,rdfs#class)  the 2 classes have no common subtype/instance; in WebKB, use the link '!'
     subtype:  pm#complement_class (rdfs#class -> rdfs#class)  if something is not in one of the classes, then it is in the other, and vice versa; in WebKB, use the link '/'