Relation pwf#friend (DigitalSubject,SubjectRelationship)
  supertype:  attribute (Higgins_ContextObject_or_DigitalSubject,Higgins_Attribute_or_Value)  Instances of this property are used to point to one or more values (i.e. cases where (i.e. cases where the range is higgins:Value). One instance (at most) may also be used to point to a holder of metadata (an instance of higgins:Attribute) about the entire set of values.
     supertype:  Higgins_object_property (?,?)
        supertype:  Higgins_property (?,?)
           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