code: func ReplaceAtLineN(filename string, r []ReplacerAtLine, n int) error {
  	e, err := NewEdit(filename)
  	if err != nil {
  		return err
  	}
  
  	err = e.genReplaceAtLine(r, n)
  	err2 := e.Close()
  	if err != nil {
  		return err
  	}
  	return err2
  }
docs: // ReplaceAtLineN replaces a number of regular expressions mathed in r for the
  // named file, if the line is matched at the first.