anchor.diff:      (with-pov-indent
         (let ((sep (rope '|,| *pov-newline-indent*)))
           (rope (rope (pov-list-name object) '| {|)
  -              (rope *pov-newline-indent* (int-string (pov-list-length object))
  +              (rope *pov-newline-indent* (pov-list-length object)
                       sep)
                 (pov-rope-reduce (pov-list-list object) sep)
                 (rope old-indent '|}|))))))
  
positive.code: 
  (defmethod object-rope ((object pov-rgb))
    (rope '|rgb<|
          (float-string (pov-rgb-r object)) '|, |
          (float-string (pov-rgb-g object)) '|, |
          (float-string (pov-rgb-b object))
          '|>|))
  
  (defstruct (pov-rgbf (:constructor %pov-rgbf (r g b f)))
  
negative.code:   (pov-print object stream))
  
  (defmethod object-rope ((object pov-float))
    (rope (pov-float-value object)))
  
  (defstruct (pov-float-vector (:constructor %pov-float-vector (x y z)))
    "Type for a povray vector."
  