nt has described 
    an algorithm for generating offset curves for X-splines,
    where the amount of offset can vary along the length
    of the X-spline.  The algorithm relies on having expressions
    for the X-spline tangent function and this was generated 
    computationally. The algorithm involves flattening the
    X-spline to a series of straight line segments, evaluating 
    the tangent function for an X-spline at the end of each segment, 
    calculating the unit normal at each point, 
    and 