Relation dl#conceptual_relation (entity,entity)
  subtype:  immediate_relation  mediated_relation
  inverse:  conceptual_relation
  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