Relation pm#relation_to_class (*,class)
  subtype:  kind  extension_fn
  supertype:  relation_to_set_or_class (*,set_or_class)
     supertype:  relation_to_collection (*,collection)
        supertype:  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:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
        supertype:  what_relation (*)
           supertype:  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:  relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type
  supertype:  relation_to_type (*,type)
     supertype:  relation_to_collection (*,collection)