Relation pm#domain_related_relation__domain-related-thing (?,?)
  subtype:  p{pm#relation_from_a_domain pm#relation_from_a_thing_that_is_not_a_domain} p{pm#relation_to_a_domain pm#relation_to_a_thing_that_is_not_a_domain}
  type:  pm#virtual_relation_type
  supertype:  pm#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:  pm#relation__related_thing__relatedthing___related_with  type for any relation (unary, binary, ..., *-ary) and instance of pm#relation_type