Function gsl_sys::gsl_spline2d_eval_deriv_x

pub unsafe extern "C" fn gsl_spline2d_eval_deriv_x(
    interp: *const gsl_spline2d, 
    x: f64, 
    y: f64, 
    xa: *mut gsl_interp_accel, 
    ya: *mut gsl_interp_accel
) -> f64