public BodyMassIndex(Weight weight, Height height)
            {
                Debug.Assert(weight != null);
                Debug.Assert(height != null);

                this.bmi = weight / (height.Doubled()) * 703;
            }
        public Person(Height height, Weight weight)
        {
            if (height == null) { throw new ArgumentNullException("height"); }
            if (weight == null) { throw new ArgumentNullException("weight"); }

            this.height = height;
            this.weight = weight;
        }