I see now -- looking at the patents, the extra layer added for phase-shifting   
   can be added to all the zones, and can have a slope to it, because after all   
   you need a slightly different phase shift at the outer edge of a zone than at   
   the inner edge.  That'   
   s why it looks so much like a refractive Fresnel lens.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)