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