anchor.diff:    (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
  +  (transitions (make-weak-hash-table :weakness :value :test #'eq)
  +               :read-only t :type hash-table)) ; V8-like links to other molds
   
   (define-print-object ((object mold) :identity nil)
     (format t "on ~A" (mold-lineage object)))
  
positive.code: 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)))
  
  ;;;
  ;;; Mold API -- Retrieval and Automatic Creation of Molds
  
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,
  