anchor.diff:  (defun read-int64-carefully (buffer index limit)
     (declare (type vector-index index limit))
     (let ((new-index (+ index 8)))
  +    (declare (type vector-index new-index))
       (if (> new-index limit)
           (error 'data-exhausted)
           (values (nibbles:sb64ref/le buffer index) new-index))))
  
positive.code:   (declare (type octet-vector buffer)
             (type vector-index index))
    (let ((new-index (+ index 8)))
      (when (> new-index limit) (error 'data-exhausted))
      (values (nibbles:ieee-double-ref/le buffer index) new-index)))
  
  
negative.code: (defun read-uint64-carefully (buffer index limit)
    (declare (type vector-index index limit))
    (let ((new-index (+ index 8)))
      (declare (type vector-index new-index))
      (if (> new-index limit)
          (error 'data-exhausted)
          (values (nibbles:ub64ref/le buffer index) new-index))))
  