Relation pm#subdomain_of_object (?,domain)
  supertype:  relation_to_a_domain__sub-domain_or_object-subdomain (?,domain)
     supertype:  domain_related_relation__domain-related-thing (?,?)
        supertype:  relation_playing_a_special_role (*)  this type permits to categorize relations according to their roles ; this is a traditional but quite subjective 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