anchor.diff:  (defun rope-iterator-next (itr)
     (let ((top (car (rope-iterator-stack itr)))
           (i (rope-iterator-i itr)))
  -    (assert (or (stringp top)
  -                (null top)))
  +    (declare (type (or null string) top))
       (cond
         ((null top) nil)
         ((= i (length top))
  
positive.code: 
  ;;; Iteration ;;;
  (defstruct rope-iterator
    (i 0)
    (stack nil))
  
  (defun rope-iterator-push (itr rope)
    (declare (type rope rope))
  
negative.code:     string))
  
  (defun rope-subrope (rope start end)
    (declare (type fixnum start end))
    (labels ((helper-string (rope start end)
               (make-array (- end start)
                           :element-type (array-element-type rope)
  