anchor.diff:    (awhen (find-mold-tree parents)
       (find-mold-by-transition it properties)))
   
  +;;;
   ;;; Creating molds
  -
  +;;;
   (defun ensure-mold-tree (parents)
     "Returns the mold tree for PARENTS, creating and caching a new one if necessary."
     (or (find-mold-tree parents)
  
positive.code:                          :properties (cons property-name
                                             (mold-properties mold))))))
  
  (defun find-mold-by-transition (start-mold goal-properties)
    "Searches the transition tree from START-MOLD to find the mold containing
  GOAL-PROPERTIES, returning that mold if found, or NIL on failure."
  
negative.code: ;; All other molds are listed as 'transitions' of these toplevel molds. See the transitions slot
  ;; on the mold struct. Each transition represents a new set of properties.
  
  ;;;
  ;;; Transitions
  ;;;
  (defun ensure-transition (mold property-name)
    "Returns the transition from MOLD indexed by PROPERTY-NAME, creating and
  linking a new one if necessary."
  