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