Relation dl#mediated_relation (entity,entity)
  subtype:  present_at  q_present_at  happens_at  overlaps  sibling_part  proper_part  generic_location  depends_on  host
  inverse:  mediated_relation
  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