anchor.diff:  
   (defun map-as-undo-vector--update (m x y)
     (declare (fixnum x))
  +  (when (eq m BTV_empty_map)
  +    (setq m (make-map-as-undo-vector (map-as-undo-vector--initial-vector) nil)))
     (let ((m (map-as-undo-vector-assure-current m)))
       ;;(incf *map-as-undo-vector-set-count*)
       (let ((vec (map-as-undo-vector--vector m)))
  
positive.code: (defparameter *map-as-undo-vector-alist-elt-ref-count* 0)
  (defparameter *map-as-undo-vector-set-count* 0)
  
  
  (defun map-as-undo-vector-assure-current (m)
    (if (map-as-undo-vector-current? m)
  
negative.code: 
  ;;; The Vector interface functions
  
  (defun BTV_numItems (m)
    (let ((m (map-as-undo-vector-assure-current m)))
      (loop for y across (map-as-undo-vector--vector m)
  