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