Relation p3p#extOptional__extoptional__optional (?,?)  A p3p processor is not required to understand the value
       of the only property of the value of this property.
  supertype:  pm#P3P_relation (?,?)
     supertype:  pm#relation_coming_from_an_ontology (*)
        supertype:  pm#relation_for_an_application (*)  relation introduced by an application
           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
              supertype:  pm#relation__related_thing__relatedthing___related_with (*)  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type

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