Require non const accessor to curves inside piecewise

I have several use cases where I have a piecewise curve and I need to replace one of the curve inside the piecewise with another curve (defined on the same time interval). This is not possible with the current API without creating another piecewise curve.

I see two way to do that : make the accessors curve_at_time and curve_at_index return non const reference or add new method to do that.

The first method is dangerous as the user could completely screw the time intervals of the curves (but this is the behaviour of my current code based on hpp_spline).

Which implementation do you prefer ?