anchor.diff:                           :accessor read-buffer-position)
      ;; Current offset into the buffer.
      (read-buffer-offset :initarg :read-buffer-offset
  -                       :initform 0
  +                       :initform -1
                          :accessor read-buffer-offset)
      ;; File size
      (read-buffer-size :initarg :read-buffer-size
  
positive.code:   (error "Feature not implemented: ~a" 'sys.gray:stream-file-position))
  
  (defmethod sys.gray:stream-file-length ((stream fat32-file-stream))
    (error "Feature not implemented: ~a" 'sys.gray:stream-file-length))
  
  ;;;; testing
  #|
  
negative.code:          do (progn
                (setf buffer (mezzano.fat32-file-system:read-file file (partition host)
                                                                  (fat32-structure host))
                      read-buffer-position (* (fat32-bytes-per-sector fat32)
                                              (virtual-dir-first-cluster file))
                      read-buffer-size (virtual-dir-file-size file))
                (return))
           finally
  