Relation pm#Higgins_object_property (?,?)
  supertype:  Higgins_property
  subtype:  contextRelationship (ContextObject,ContextRelationship)
  subtype:  contextId__contextid (Higgins_Relationship,ContextId)
     subtype:  contextURI (Higgins_Relationship,ContextId)
  subtype:  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.
     subtype:  uniqueIdentifier (DigitalSubject -> String)
     subtype:  relation (Higgins_ContextObject_or_DigitalSubject,SubjectRelationship)  A directed relation between two Digital Subjects
        subtype:  correlation (Higgins_ContextObject_or_DigitalSubject,SubjectRelationship)  A directed relation between two Digital Subjects believed to represent the same underlying Entity
     subtype:  firstname (Person,NormalizedStringSimpleAttribute)
     subtype:  surname (Person,NormalizedStringSimpleAttribute)
     subtype:  postalAddress (Person,PostalAddress)
     subtype:  streetAddress (PostalAddress,String)
     subtype:  city (PostalAddress,String)
     subtype:  state (Higgins_ContextObject_or_DigitalSubject,String)
     subtype:  postalCode (PostalAddress,String)
     subtype:  country (PostalAddress,String)
     subtype:  friend (DigitalSubject,SubjectRelationship)
  subtype:  complexMetadata__complexmetadata (Higgins_DigitalSubject_or_Value,?)  Used as the super-Property of some ObjectProperties of higgins:ValueAndMetadata or higgins:DigitalSubject to indicate that these properties are considered to be metadata about the ValueAndMetadata (or DigitalSubject)
     subtype:  source (Value,SubjectRelationship)
     subtype:  creator (Value,SubjectRelationship)
     subtype:  timeSpan (Higgins_DigitalSubject_or_Value,TimeSpan)