code: func (r Registry) Register(name string, factory PluginFactory) error {
  	if _, ok := r[name]; ok {
  		return fmt.Errorf("a plugin named %v already exists", name)
  	}
  	r[name] = factory
  	return nil
  }
docs: // Register adds a new plugin to the registry. If a plugin with the same name
  // exists, it returns an error.