code: func targetHost(c CommandLine, api libmachine.API) (string, error) {
  	if len(c.Args()) == 0 {
  		defaultExists, err := api.Exists(defaultMachineName)
  		if err != nil {
  			return "", fmt.Errorf("Error checking if host %q exists: %s", defaultMachineName, err)
  		}
  
  		if defaultExists {
  			return defaultMachineName, nil
  		}
  
  		return "", ErrNoDefault
  	}
  
  	return c.Args()[0], nil
  }
docs: // targetHost returns a specific host name if one is indicated by the first CLI
  // arg, or the default host name if no host is specified.