f.update();
checkValues("Natural spline", f, generic_x, generic_y);
// cached second derivative
for (int i=0; i<n; i++) {
interpolated = f.secondDerivative(generic_x.get(i));
error = interpolated - generic_natural_y2.get(i);
assertFalse("Natural spline interpolation "
+"second derivative failed at x="+generic_x.get(i)
+"\n interpolated value: "+interpolated
+"\n expected value: "+generic_natural_y2.get(i)