anchor.diff:  (defparameter *map-as-undo-vector-alist-elt-ref-count* 0)
   (defparameter *map-as-undo-vector-set-count* 0)
   
  +(defparameter BTV_empty_map (make-map-as-undo-vector (map-as-undo-vector--initial-vector) nil))
   
   (defun map-as-undo-vector-assure-current (m)
     (if (map-as-undo-vector-current? m)
  
positive.code: 
  (defun map-as-undo-vector--update (m x y)
    (declare (fixnum x))
    (let ((m (map-as-undo-vector-assure-current m)))
      ;;(incf *map-as-undo-vector-set-count*)
      (let ((vec (map-as-undo-vector--vector 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)
  