anchor.diff:                      :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))
  
positive.code: (defclass local-variable-reference (variable-reference)
    ())
  
  (defclass free-variable-reference (variable-reference)
    ())
  
  
negative.code: 
  ;;;; ** Public Entry Point
  
  (defun walk-form (form &optional (parent nil) (env (make-walk-env)))
    "Walk FORM and return a FORM object."
    (funcall (find-walker-handler form) form parent env))
  
  (defun make-walk-env (&optional lexical-env)
    (let ((walk-env '()))
      (when lexical-env
        (dolist (var (lexical-variables lexical-env))
          (setf walk-env (register walk-env :lexical-let var t))))
      walk-env))
  
  ;;;; This takes a Common Lisp form and transforms it into a tree of
  ;;;; FORM objects.
  