anchor.diff:  ;;;
   ;;; Molds
   ;;;
  -(defvar *molds* (make-hash-table :test 'equal)
  +(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.")
   
  
positive.code: 
  (defstruct (mold
               (:predicate moldp)
               (:constructor make-mold (lineage properties)))
    "Also known as 'backend classes', molds are hidden caches which enable
  Sheeple to use class-based optimizations yet keep its dynamic power."
    (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
  
negative.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)) mold)
          (aconsf (mold-transitions mold) property-name (make-weak-pointer it)))))
  
  (defun ensure-mold (parents &optional (properties #()))
  