anchor.diff:                       (depth-mask boolean)
                        (depth-range boolean)
                        (depth-clamp boolean)
  +                     (clear-color boolean)
  +                     (cull-face boolean)
  +                     (front-face boolean)
  +                     (viewport boolean)
                        (color-mask-indices list)
                        (tex-unit-ids list)
                        (buffer-targets list)
  
positive.code:       (setf (depth-clamp context) (depth-clamp context)))
  
      ;; cull-face
      (setf (cull-face context) (cull-face context))
  
      ;; front-face
      (setf (front-face context) (front-face context))
  
      ;; clear-color
      (setf (clear-color context) (clear-color context))
  
      ;; viewport
      (cepl.viewports::%set-current-viewport context current-viewport)
  
      ;; color-masks
      (loop :for index :in color-mask-indices
  
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)))
  