Relation p3p#data (Statement,DataElement)  the data to be transferred or inferred
  subtype:  optionalData (pm)  the data to be optionaly transferred or inferred
  supertype:  P3P_relation (?,?)
     supertype:  relation_coming_from_an_ontology (*)
        supertype:  relation_for_an_application (*)  relation introduced by an application
           supertype:  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:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type