anchor.diff:  
   (defstruct (mold
                (:predicate moldp)
  -             (:constructor make-mold (lineage properties)))
  +             (:constructor make-mold (lineage properties &optional back)))
     "Also known as 'backend classes', molds are hidden caches which enable
   Sheeple to use class-based optimizations yet keep its dynamic power."
  +  (back        nil :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 (or simple-vector hash-table)) ; Direct properties
  
positive.code:   (check-type property-name property-name)
    (or (find-transition mold property-name)
        (aprog1 (make-mold (mold-lineage mold)
                           (hv-cons property-name (mold-properties mold)))
          (aconsf (mold-transitions mold) property-name (make-weak-pointer it)))))
  
  (defun ensure-mold (parents &optional (properties #()))
  
negative.code: ;;;
  ;;; Molds
  ;;;
  (defvar *molds* (make-weak-hash-table :test 'equal :weakness :value)
    "Maps parent lists to their corresponding molds. This is the global entry
  point to Sheeple's backend class system.")
  
  