anchor.diff:                              buffer (cdr buffer))))
   
                      (t
  -                    (setf (cdr compute) nil)))
  +                    (setf (cdr buffer) 'eof)))
                (values value more)))
   
             (t
  
positive.code:     (do-iterator (value iter :result vector)
        (vector-push-extend value vector))))
  
  (defclass lookahead-iterator ()
    ((compute
      :initform (cons t nil)
      :initarg :compute)
     (buffer
      :initform (cons 'tail 'tail)
      :initarg :buffer
      :type cons))
  
negative.code: 
  (defclass iterator ()
    ((compute
      :initform (required)
      :initarg :compute
      :documentation
      "A function that returns the next value."))
  