Relation dl#happens_at__happensat (perdurant,time_interval)
  inverse:  time_of_happening_of
  supertype:  mediated_relation (entity,entity)
     supertype:  conceptual_relation (entity,entity)
        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