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