code: func (w *WrapWriter) Write(p []byte) (n int, err error) {
  	n, err = Wrap(w, p, w.off, w.wrapAt)
  	w.off = (w.off + int64(n)) % w.wrapAt
  	return n, err
  }
docs: // Write writes p starting at the current offset, wrapping when it reaches the end.
  // The current offset is shifted forward by the amount written.