Relation higgins#validFrom__validfrom (TimeSpan,date)
  supertype:  simpleMetadata (Value,DigitalSubject)  Used as the super-Property of some DatatypeProperties whose domain is higgins#ValeAndMetadata or higgins#DigitalSubject to indicate that these properties are considered to be metadata about the ValueAndMetadata (or DigitalSubject)
     supertype:  Higgins_datatype_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