anchor.diff:        (setf (depth-clamp context) (depth-clamp context)))
   
       ;; cull-face
  -    (setf (cull-face context) (cull-face context))
  +    (when cull-face
  +      (setf (cull-face context) (cull-face context)))
   
       ;; front-face
  -    (setf (front-face context) (front-face context))
  +    (when front-face
  +      (setf (front-face context) (front-face context)))
   
       ;; clear-color
  -    (setf (clear-color context) (clear-color context))
  +    (when clear-color
  +      (setf (clear-color context) (clear-color context)))
   
       ;; viewport
  -    (cepl.viewports::%set-current-viewport context current-viewport)
  +    (when viewport
  +      (cepl.viewports::%set-current-viewport context current-viewport))
   
       ;; color-masks
       (loop :for index :in color-mask-indices
  
positive.code:                         ,depth-mask
                          ,depth-range
                          ,depth-clamp
                          nil
                          nil
                          nil
  
negative.code:                                         depth-test-function
                                          depth-mask
                                          depth-range
                                          depth-clamp
                                          clear-color
                                          cull-face
                                          front-face
                                          viewport)
                                         &body body)
    (assert (member program '(t nil)))
    (assert (member stencil '(t nil)))
  