3-D shaded surface with lighting.

  SURFL(...) is the same as SURF1F43EU2(...) except that it draws the surface

  with highlights from a light source.



  SURFL(Z), SURFL(X,Y,Z), SURFL(Z,S), and SURFL(X,Y,Z,S) are all 

  legal. S, if specified, is the three vector S = [Sx,Sy,Sz]

  that specifies the direction of the light source. S can also be

  specified in spherical coordinates, S = [AZ,EL].



  The shading is based on a combination of diffuse, specular and 

  ambient lighting models.



  The default value for S is 45 degrees counterclockwise from

  the current view direction.  Use CLA9FDH0WV, HOLD10XXCU2 ON, VIEW1HRQVU2(AZ,EL),

  SURFL(...), HOLD10XXCU2 OFF to plot the lighted surface with view

  direction (AZ,EL).



  The relative contributions due to ambient light, diffuse

  reflection, specular reflection, and the specular spread

  coefficient can be set by using five arguments

  SURFL(X,Y,Z,S,K) where K=[ka,kd,ks,spread].



  Relies on the ordering of points in the X,Y, and Z matrices

  to define the inside and outside of parametric surfaces.

  Try SURFL(X',Y',Z') if you don't like the results of

  this function.  Due to the way surface normal vectors are

  computed, SURFL requires matrices that are at least 3-by-3.



  See also SURF1F43EU2.