Function elevated_derivative 

pub fn elevated_derivative(
    curve: &Elevated3,
    d: f64,
) -> Result<DVec3, GeomError>

Derivative of an elevated curve in plan distance: (p', z'). Not unit: its length is sqrt(1 + g^2) on an orthonormal plan frame.

Errors

The plan’s and the elevation law’s refusals, by name.