anchor.diff:  `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
  +  (with-%ptree-slots (nodes) ptree
       (flet ((fetch (name) (or (gethash name nodes)
                                (setf (gethash name nodes)
                                      (make-node-instance :name name)))))
  
positive.code: 
  (defun make-ptree ()
    "Create a ptree instance."
    (make-ptree-instance))
  
  (defun ptree-fn (name args function ptree)
    "Define a ptree node labeled `name' (a symbol) in `ptree' (a ptree instance).
  
negative.code: ;;;
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  
  (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
      (maphash (lambda (name node)
                 (declare (ignore name))
                 (check-node node))
  