Relation pm#trichotomizing_relation_on_real_number (quantity,quantity)
  supertype:  trichotomizing_relation  this category only serves structuration purposes: it is instance of pm#trichotomizing_relation_type which is not instance of pm#class_of_inheritable_relation_type
  subtype:  less_than (quantity,quantity)
  subtype:  greater_than__greaterthan (quantity,quantity)
  subtype:  less_than_or_equal (quantity,quantity)
  subtype:  greater_than_or_equal (quantity,quantity)