anchor.diff:  (defclass local-variable-reference (variable-reference)
     ())
   
  +(defclass local-lexical-variable-reference (local-variable-reference)
  +  ()
  +  (:documentation "A reference to a local variable defined in the
  +  lexical envorinment outside of the form passed to walk-form."))
  +
   (defclass free-variable-reference (variable-reference)
     ())
   
  
positive.code: 
  ;;;; ** Public Entry Point
  
  (defun walk-form (form parent env)
    "Walk FORM and return a FORM object."
    (funcall (find-walker-handler form) form parent env))
  
  ;;;; This takes a Common Lisp form and transforms it into a tree of
  ;;;; FORM objects.
  
negative.code:                     :parent parent :source form))
      ((lookup env :let form)
       (make-instance 'local-variable-reference :name form
                      :parent parent :source form))
      ((lookup env :lexical-let form)
       (make-instance 'local-lexical-variable-reference :name form
                      :parent parent :source form))
      ((lookup env :symbol-macrolet form)
       (walk-form (lookup env :symbol-macrolet form) parent env))
  