#include <iostream>

class Interval
{
 private:
   enum {normal, empty, whole} flag;

   double lower, upper;

 public:
   Interval(double low, double up)
      : flag(normal), lower(low), upper(up)
   {};

   friend Interval addByValue(Interval a, Interval b);
   friend Interval addByConstRef(const Interval &a, const Interval &b);
   friend Interval addByRef(Interval &a, Interval &b);

   inline friend Interval addByValueInline(Interval a, Interval b)
   { return Interval(a.lower+b.lower, a.upper+b.upper); };

   inline friend Interval addByRefInline(Interval &a, Interval &b)
   { return Interval(a.lower+b.lower, a.upper+b.upper); }

   inline friend Interval addByConstRefInline(const Interval &a, const Interval &b)
   { return Interval(a.lower+b.lower, a.upper+b.upper); };

   friend std::ostream& operator<<(std::ostream &os, const Interval &obj);
};