anchor.diff:  If no such mold exists, returns NIL."
     (check-type mold mold)
     (check-type property-name property-name)
  -  (awhen (cdr (assoc property-name (mold-transitions mold) :test 'eq))
  -    (weak-pointer-value it)))
  +  (values (gethash property-name (mold-transitions mold))))
   
   ;;;
   ;;; Mold API -- Retrieval and Automatic Creation of Molds
  
positive.code:   (back        nil :read-only t :type (or null mold)) ; Back pointer
    (lineage     nil :read-only t :type lineage) ; A common cache of parent stuff
    (properties  nil :read-only t :type simple-vector) ; Direct properties
    (transitions nil :type list))         ; V8-like links to other molds
  
  (define-print-object ((object mold) :identity nil)
    (format t "on ~A" (mold-lineage object)))
  
negative.code:   (or (find-transition mold property-name)
        (aprog1 (make-mold (mold-lineage mold)
                           (vector-cons property-name (mold-properties mold)) mold)
          (setf (gethash property-name (mold-transitions mold)) it))))
  
  (defun ensure-mold (parents &optional (properties #()))
    "Returns the mold with properties PROPERTIES of the mold for PARENTS,
  