anchor.diff:                             (let ((cr (pointer cr)))
                                (cairo-set-source-rgb cr 0.1 0.7 0.0)
                                (cairo-paint cr)
  -                             (dolist (actor actors) (draw actor cr))
  +                             (dolist (actor (actors environment))
  +                               (draw actor cr))
                                (cairo-destroy cr)
                                t)))
          (gtk-container-add window box)
  
positive.code:   (cairo-set-font-size cr 10.0)
    (cairo-show-text cr text))
  
  (defun make-herd (num-sheep)
    (let (herd)
      (dotimes (i num-sheep)
        (push (make-instance 'sheep :herd nil) herd))
      (dolist (sheep herd)
        (setf (herd sheep) herd))
      herd))
  
  (defun distance (one two)
    (sqrt (+ (expt (- (x one) (x two)) 2)
  
negative.code:    (name
      :initarg :name
      :accessor name
      :initform 'unnamed)
     (environment
      :initarg :environment
      :accessor environment
      :initform nil)))
  
  (defclass mover (actor)
    ((direction-fn
  