code: def normal_surface_single(obj, uv, normalize):
      
      # Take the 1st derivative of the surface
      skl = obj.derivatives(uv[0], uv[1], 1)
  
      point = skl[0][0]
      vector = linalg.vector_cross(skl[1][0], skl[0][1])
      vector = linalg.vector_normalize(vector) if normalize else vector
  
      return tuple(point), tuple(vector)
docs: Evaluates the surface normal vector at the given (u, v) parameter pair.
  
      The output returns a list containing the starting point (i.e. origin) of the vector and the vector itself.
  
      :param obj: input surface
      :type obj: abstract.Surface
      :param uv: (u,v) parameter pair
      :type uv: list or tuple
      :param normalize: if True, the returned normal vector is converted to a unit vector
      :type normalize: bool
      :return: a list in the order of "surface point" and "normal vector"
      :rtype: list