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