code: func ContainString(filename, s string) (bool, error) {
  	f, err := os.Open(filename)
  	if err != nil {
  		return false, fmt.Errorf("ContainString: %s", err)
  	}
  	defer f.Close()
  
  	buf := bufio.NewReader(f)
  
  	for {
  		line, err := buf.ReadString('\n')
  		if err == io.EOF {
  			break
  		}
  		if strings.Contains(line, s) {
  			return true, nil
  		}
  	}
  	return false, nil
  }
docs: // ContainString returns whether the named file contains the string s. The
  // return value is a boolean.