anchor.diff:  
   (defun make-rectangular-tile (design width height)
     (make-instance 'rectangular-tile
  -    :width width
  +    :width  width
       :height height
       :design design))
   
  -
   ;;;
   
   (defclass in-compositum (design)
  -  ((ink :initarg :ink :reader compositum-ink)
  +  ((ink  :initarg :ink  :reader compositum-ink)
      (mask :initarg :mask :reader compositum-mask)))
   
   (defmethod print-object ((object in-compositum) stream)
  
positive.code:         (t
           (make-instance 'standard-opacity :value value))))
  
  (defvar +transparent-ink+ 
      (make-instance 'standard-opacity :value 0))
  
  ;;;;
  ;;;; 13.6 Indirect Inks
  ;;;;
  
  (defclass indirect-ink (design) ())
  
  (defvar +foreground-ink+ (make-instance 'indirect-ink))
  (defvar +background-ink+ (make-instance 'indirect-ink))
  
  (defmethod print-object ((ink (eql +foreground-ink+)) stream)
    (format stream "#.~S" '+foreground-ink+))
  
  (defmethod print-object ((ink (eql +background-ink+)) stream)
    (format stream "#.~S" '+background-ink+))
  
  ;;;;
  ;;;; 13.7 Flipping Ink
  ;;;;
  
negative.code: ;; (make-opacity 1) = +foreground-ink+ = +everywhere+
  ;;
  ;; --GB
  
  ;; It might be handy to have the equivalent of parent-relative
  ;; backgrounds. We can specify new indirect inks:
  