Can implement a 'not-a-knot' boundary condition like Python?

[Odinman]

I found that the default result is the same as Python(scipy.interpolate.CubicSpline)'s natural boundary condition.
But I don't know how to match the default result of Python(with default boundary condition not-a-knot)

[ttk592]

I've just implemented the not-a-knot condition, let me know if you can match that with Python as well?
You can set it up like this:

    tk::spline s;
    s.set_boundary(tk::spline::not_a_knot, 0.0,
                   tk::spline::not_a_knot, 0.0);
    s.set_points(...);