SURFDERIVEVAL: Compute the derivatives of a B-spline surface

 usage: skl = surfderiveval (n, p, U, m, q, V, P, u, v, d) 

  INPUT: 

        n+1, m+1 = number of control points
        p, q     = spline order
        U, V     = knots
        P        = control points
        u,v      = evaluation points
        d        = derivative order

  OUTPUT:

        skl (k+1, l+1) =  surface differentiated k
                          times in the u direction and l
                          times in the v direction

 Adaptation of algorithm A3.8 from the NURBS book

Package: nurbs