Relation higgins#relation (Higgins_ContextObject_or_DigitalSubject,SubjectRelationship)  A directed relation between two Digital Subjects
  supertype:  attribute  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.
  subtype:  correlation (Higgins_ContextObject_or_DigitalSubject,SubjectRelationship)  A directed relation between two Digital Subjects believed to represent the same underlying Entity