func fileSource(filename string, i int) string {
	return fmt.Sprintf("%s:%d", filename, i)
}
func fileSource(filename string, i int) string {
	return fmt.Sprintf("%s:%d", filename, i)
}
func fileSource ( filename string , i int ) string { return fmt . Sprintf ( " " , filename , i ) }
// fileSource returns a source ID for the i-th target group in the file.
fileSource returns a source ID for the i - th target group in the file .