static void Main(string[] args)
        {
            box a = new box(); //first box

            a.h = 4;           //height
            a.l = 2;           //length
            a.w = 9;           //width

            box b = new box(); //second box

            b.w = 5;
            b.l = 3;
            b.h = 6;
            box z = new box(); // another object to receive value from operator

            z = a + b;
            Console.WriteLine("NEW VOLUME = " + z.volume());
        }