anchor.diff:  (defparameter *map-as-undo-harray-alist-elt-ref-count* 0)
   (defparameter *map-as-undo-harray-set-count* 0)
   
  +;;; (list MapSTHashtable::*map-as-undo-harray-undo-count* MapSTHashtable::*map-as-undo-harray-copy-count*)
  +
   (defvar *undefined* '(:|None|))
   (defun defined? (val) 
     (not (eq val *undefined*)))
  
positive.code:       ;(incf *map-as-undo-harray-set-count*)
        (let* ((last-undo-list (map-as-undo-harray--undo-list m))
  	     (old-val (gethash x table *undefined*)))
  	(if (eq y old-val)
  	    m				; Optimize special case
  	  (progn
              (when (member table *maphash-htables* :test 'eq)
  
negative.code:       m
      (let ((undo-list (nreverse (map-as-undo-harray--undo-list m)))
  	  (table (map-as-undo-harray--harray m)))
        ; (incf *map-as-undo-harray-undo-count* (length undo-list))
        ; (incf *map-as-undo-harray-copy-count*)
        (when *break-on-non-single-threaded-updates?*
  	(break "Non-single-threaded update!"))
        (let ((new-table (copy-hash-table table)))
  