public SplineFunction AddSpline(IFunction spline)
 {
     if (Splines.Count == 0 ||
         (Splines.Count != 0 && spline.Start () == Splines [Splines.Count - 1].End ())) {
         Splines.Add (spline);
         return this;
     }
     else
         throw new ArgumentException ();
 }