Relation pm#sub_collection__subcollection (collection,collection)  a partial order relation
  supertype:  relation_between_collections  part  partial_ordering_relation
  instance of:  partial_ordering_relation_type
  subtype:  ending_collection (collection,collection)
     subtype:  final_segment__finalsegment (collection,collection)  the second collection is a final segment of the 1st
  subtype:  sublist__final_segment_of__finalsegmentof (list,list)  USE pm#final_segment INSTEAD OF THIS RELATION TYPE; "sublist" is a misleading name; "final_segment_of" is better