Relation pm#relation_to_type (*,type)
  supertype:  relation_to_collection
  subtype:  relation_to_class (*,class)
     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