Relation pm#author (pm#description,pm#causal_entity)
  supertype:  pm#relation_from_description_content/medium/container  dc#Creator
  equal:  tap#has_author (pm)
  subtype:  pm#main_author (pm#description,pm#causal_entity)
  subtype:  pm#co-author (pm#description,pm#causal_entity)