deferred class interface COMPARABLE
   --
   -- All classes handling COMPARABLE objects with a total order relation
   -- should inherit from this class.
   --

feature(s) from COMPARABLE   is_equal (other: like Current): BOOLEAN
      -- Is other attached to an object considered equal to
      -- current object ?

      require
         other /= Void
      ensure
         trichotomy: Result = (not (Current < other) and not (other < Current));
         generating_type = other.generating_type implies Result = other.is_equal(Current)

   infix "<" (other: like Current): BOOLEAN
      -- Is Current strictly less than other?

      require
         other_exists: other /= Void
      ensure
         asymmetric: Result implies not (other < Current)

   infix "<=" (other: like Current): BOOLEAN
      -- Is Current less than or equal other?

      require
         other_exists: other /= Void
      ensure
         definition: Result = (Current < other or is_equal(other))

   infix ">" (other: like Current): BOOLEAN
      -- Is Current strictly greater than other?

      require
         other_exists: other /= Void
      ensure
         definition: Result = (other < Current)

   infix ">=" (other: like Current): BOOLEAN
      -- Is Current greater than or equal than other?

      require
         other_exists: other /= Void
      ensure
         definition: Result = (other <= Current)

   in_range (lower, upper: like Current): BOOLEAN
      -- Return true if Current is in range [lower..upper]

      ensure
         Result = (Current >= lower and Current <= upper)

   compare (other: like Current): INTEGER
      -- If current object equal to other, 0
      -- if smaller,  -1; if greater, 1.

      require
         other_exists: other /= Void
      ensure
         equal_zero: Result = 0 = is_equal(other);
         smaller_negative: Result = -1 = (Current < other);
         greater_positive: Result = 1 = (Current > other)

   three_way_comparison (other: like Current): INTEGER
      -- If current object equal to other, 0
      -- if smaller,  -1; if greater, 1.

      require
         other_exists: other /= Void
      ensure
         equal_zero: Result = 0 = is_equal(other);
         smaller_negative: Result = -1 = (Current < other);
         greater_positive: Result = 1 = (Current > other)

   min (other: like Current): like Current
      -- Minimum of Current and other.

      require
         other /= Void
      ensure
         Result <= Current and then Result <= other;
         compare(Result) = 0 or else other.compare(Result) = 0

   max (other: like Current): like Current
      -- Maximum of Current and other.

      require
         other /= Void
      ensure
         Result >= Current and then Result >= other;
         compare(Result) = 0 or else other.compare(Result) = 0



end of deferred COMPARABLE