code: func Register(name string, fn Factory) {
  	if registry == nil {
  		registry = make(map[string]Factory)
  	}
  
  	if registry[name] != nil {
  		panic(fmt.Errorf("Command %q is already registered", name))
  	}
  	registry[name] = fn
  }
docs: // Register adds a new CLI sub-command to the registry.