anchor.diff:    (multiple-value-call #'values
       (color-rgb (indirect-ink-ink +foreground-ink+))
       (opacity-value color)))
  +
  +;;; Regions
  +(defmethod design-ink ((design region) x y)
  +  (if (region-contains-position-p design x y)
  +      (design-ink +foreground-ink+ x y)
  +      +transparent-ink+))
   
   ;;;;
   ;;;; 13.7 Flipping Ink
  
positive.code:         (symbol-value dynamic-variable-symbol)
          default-ink)))
  
  (defmethod design-ink ((ink indirect-ink) x y)
    (let ((ink (indirect-ink-ink ink)))
      (design-ink ink x y)))
  
negative.code:    (blue  :initarg :blue
            :initform 0
            :type (real 0 1))))
  
  (defmethod opacity-value ((ink standard-color))
    1.0)
  
  (defmethod color-rgb ((color standard-color))
    (with-slots (red green blue) color
  