code: func (r *resolver) ResolveItemAt(i int, it *config.Config) error {
  	msgV := archetypeInstance(r.singleType)
  	if err := r.resolveItem(msgV.Interface().(proto.Message), it.Content, it.FormatSpec.Formatter); err != nil {
  		return err
  	}
  	r.multiDest.Index(i).Set(msgV)
  	return nil
  }
docs: // ResolveItemAt implements cfgclient.MultiResolver.