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