anchor.diff:  ;;;
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   
  -(defslots ptree ()
  +(defslots %ptree ()
     ((nodes :initform (make-hash-table :test #'eq) :type hash-table)))
   
   (defun check-ptree (ptree)
     "Verify that all nodes have been defined with an associated
   function. If not, `ptree-undefined-function-error' is signaled."
  -  (with-ptree-slots (nodes) ptree
  +  (with-%ptree-slots (nodes) ptree
       (maphash (lambda (name node)
                  (declare (ignore name))
                  (check-node node))
  
positive.code: 
  If the node is already computed, return the computed result. A node is
  never computed twice."
    (with-ptree-slots (nodes) ptree
      (let1 root (gethash name nodes)
        (unless root
          (error 'ptree-undefined-function-error :name name))
  
negative.code: `function' is the function associated with this node. The arguments
  passed to `function' are the respective results of the child node
  computations."
    (with-%ptree-slots (nodes) ptree
      (flet ((fetch (name) (or (gethash name nodes)
                               (setf (gethash name nodes)
                                     (make-node-instance :name name)))))
  