Relation pm#relation_to_set_or_class (*,set_or_class)
  subtype:  closed_on  reflexive_on  irreflexive_on  partial_ordering_on  total_ordering_on  trichotomizing_on  equivalence_relation_on  relation_to_class  relation_to_another_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