Relation pm#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
  supertype:  binary_relation_with_particular_mathematical_property
  instance of:  trichotomizing_relation_type
  subtype:  trichotomizing_relation_on_real_number (quantity,quantity)
     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)