code: func checkShortIDs(cfg config.Wrapper) error {
  	exists := make(map[protocol.ShortID]protocol.DeviceID)
  	for deviceID := range cfg.Devices() {
  		shortID := deviceID.Short()
  		if otherID, ok := exists[shortID]; ok {
  			return fmt.Errorf("%v in conflict with %v", deviceID, otherID)
  		}
  		exists[shortID] = deviceID
  	}
  	return nil
  }
docs: // checkShortIDs verifies that the configuration won't result in duplicate
  // short ID:s; that is, that the devices in the cluster all have unique
  // initial 64 bits.