code: func (d *Dataset) Fprintf(writer io.Writer, format string) *Dataset {
  	fn := func(r io.Reader) error {
  		w := bufio.NewWriter(writer)
  		defer w.Flush()
  		return util.Fprintf(w, r, format)
  	}
  	return d.Output(fn)
  }
docs: // Fprintf formats using the format for each row and writes to writer.