anchor.diff:                  (g-timeout-add (floor (* 1000 +time-step+)) #'update-all))
                 (update-all (&optional widget)
                   (declare (ignore widget))
  -                (dolist (actor actors) (update actor))
  +                (update environment)
                   (gtk-widget-queue-draw drawing-area)
                   runningp))
          (g-signal-connect window "destroy" (lambda (widget)
  
positive.code:                            (let ((cr (pointer cr)))
                               (cairo-set-source-rgb cr 0.1 0.7 0.0)
                               (cairo-paint cr)
                               (dolist (actor actors) (draw actor cr))
                               (cairo-destroy cr)
                               t)))
         (gtk-container-add window box)
  
negative.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)
  