Relation pm#relation_to_class (*,rdfs#class)
  supertype:  pm#relation_to_set_or_class  pm#relation_to_type
  subtype:  pm#kind__type___class___instance_of__instanceof (?,rdfs#class)  the '^' link in the FT notation
  subtype:  sumo#extension_fn__extensionfn (sumo#Attribute -> sumo#class)  a unary_function that maps an attribute into the class whose condition for membership is the attribute