code: func filterUnusedLogs(logs []string) (inuse []string, unused []string) {
  	for _, l := range logs {
  		if isInUse(l, logs) {
  			inuse = append(inuse, l)
  		} else {
  			unused = append(unused, l)
  		}
  	}
  	return inuse, unused
  }
docs: // filterUnusedLogs splits logs into 2 groups, the 1st group is in used logs,
  // the second group is unused logs.