Relation pm#relation_to_string (*,string)
  supertype:  relation_to_collection
  subtype:  extrinsic_relation__extrinsicrelation (?,Literal)  support for relations ranging on concrete data types
     subtype:  name (entity,Literal)  a relation between an instance of a concept and the name (instance of a concrete data type) of that concept; the name has to be a lexically meaningful name
     subtype:  identifier (entity -> Literal)  a relation between an individual of a concept and the identifier of that concept; the identifier is a concrete data type, usually non_meaningful from the lexical viewpoint; useful to map many database relationships