Relation pm#relation_for_an_application (*)  relation introduced by an application
  subtype:  pm#relation_coming_from_an_ontology
  supertype:  pm#relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective way of categorizing relations
     >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type