Relation pm#relation_from_time_to_time (pm#time_measure,pm#time_measure)
  supertype:  pm#relation_to_time
  subtype:  pm#near_time (pm#time_measure,pm#time_measure)
  subtype:  pm#before (pm#time_measure,pm#time_measure)
  subtype:  pm#after (pm#time_measure,pm#time_measure)