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: (defgeneric compose-in (ink mask))
  (defgeneric compose-out (ink mask))
  
  ;; PATTERN is just the an abstract class of all pattern-like design. 
  
  ;; For performance might consider to sort out pattern, which consists
  ;; of uniform designs only and convert them to an RGBA-image.
  
  (define-protocol-class pattern (design))
  
  (defclass indexed-pattern (design)
    ((array   :initarg :array)
     (designs :initarg :designs)))
     
  (defun make-pattern (array designs)
    (make-instance 'indexed-pattern :array array :designs designs))
  
  (defmethod pattern-width ((pattern indexed-pattern))
    (with-slots (array) pattern
      (array-dimension array 1)))
  
  (defmethod pattern-height ((pattern indexed-pattern))
    (with-slots (array) pattern
      (array-dimension array 0)))
  
  (defclass stencil (pattern)
    ((array :initarg :array)))
  
  (defun make-stencil (array)
    (make-instance 'stencll :array array))
  
  (defmethod pattern-width ((pattern stencil))
    (with-slots (array) pattern
      (array-dimension array 1)))
  
  (defmethod pattern-height ((pattern stencil))
    (with-slots (array) pattern
      (array-dimension array 0)))
  
  ;;;
  
  (defclass rectangular-tile (design)
    ((width  :initarg :width      :reader rectangular-tile-width)
  