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: (defgeneric compose-in (ink mask))
  (defgeneric compose-out (ink mask))
  
  (defclass pattern (design)
    ((array :initarg :array)
     (designs :initarg :designs)))
     
  (defun make-pattern (array designs)
    (make-instance 'pattern :array array :designs designs))
  
  (defun pattern-width (pattern)
    (nyi))
  
  (defun pattern-height (pattern)
    (nyi))
  
  (defun make-stencil (array)
    (nyi))
  
  (defclass rectangular-tile (design)
    ((width  :initarg :width      :reader rectangular-tile-width)
  
negative.code:         (t
           (make-instance 'standard-opacity :value value))))
  
  ;;;;
  ;;;; 13.7 Flipping Ink
  ;;;;
  