Function pm#on (pm#spatial_object -> pm#spatial_object)
  supertype:  pm#above (pm#spatial_object -> pm#spatial_object)
     supertype:  pm#location (pm#spatial_object,pm#spatial_object)
        supertype:  pm#relation_to_another_spatial_object (pm#spatial_object,pm#spatial_object)
           supertype:  pm#relation_from_spatial_object (pm#spatial_object,*)
              supertype:  pm#spatial_relation_from_entity_with_spatial_feature (sumo#object,*)
                 supertype:  pm#relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
                    >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
           supertype:  pm#relation_to_spatial_object (*,pm#spatial_object)  in WebKB, prefer using relations "from" a spatial object
              supertype:  pm#spatial_relation_to_entity_with_spatial_feature (*,sumo#object)
                 supertype:  pm#relation_from/to_thing_of_common_kind (*)  this type permits to categorize relations according to their signatures and hence offers (i) a concise way to set essential exclusion relations, and (ii) a systematic and easy-to-follow categorization
           supertype:  pm#where_relation__whererelation (*)  where, from/to where, ...
              supertype:  pm#wh-/how_relation (*)  this type permits to categorize relations according to the usual who/what/why/where/when/how questions ; this is a traditional but very subjective and ineffective way of categorizing relations 
                 >part of:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type