SURFDERIVCPTS: Compute control points of n-th derivatives of a NURBS surface.
 
 
usage: pkl = surfderivcpts (n, p, U, m, q, V, P, d)  
 
  INPUT:  

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

  OUTPUT: 

        pkl (k+1, l+1, i+1, j+1) = i,jth control point 
                                   of the surface differentiated k 
                                   times in the u direction and l 
                                   times in the v direction 
 
 Adaptation of algorithm A3.7 from the NURBS book

Package: nurbs