anchor.diff:    (with-slots (x1 y1 x2 y2) self
       (format sink "#<~S ~D ~D ~D ~D>" (type-of self) x1 y1 x2 y2)))
   
  -;;; ---- 2.5.5 Rectangles in CLIM --------------------------------------------------------
  +;;; ---- 2.5.5 Rectangles in CLIM ----------------------------------------------
   
   ;; protocol:
   ;;     rectangle-edges*
  
positive.code:     (and (<= x1 (coerce x 'coordinate) x2)
           (<= y1 (coerce y 'coordinate) y2))))
  
  ;;; ---- 2.5.6 Ellipses and Elliptical Arcs in CLIM --------------------------------------
  
  (defclass elliptical-thing ()
    ((start-angle :initarg :start-angle)
  
negative.code:   (multiple-value-bind (x y) (point-position point)
      (region-contains-position-p xs x y)))
  
  ;;; ---- interval sums ---------------------------------------------------------
  
  (defun isum-union* (xs ys)        (isum-op xs ys boole-ior   0 0 nil))
  (defun isum-difference* (xs ys)   (isum-op xs ys boole-andc2 0 0 nil))
  