code: func (c *rawConnection) Index(folder string, idx []FileInfo) error {
  	select {
  	case <-c.closed:
  		return ErrClosed
  	default:
  	}
  	c.idxMut.Lock()
  	c.send(&Index{
  		Folder: folder,
  		Files:  idx,
  	}, nil)
  	c.idxMut.Unlock()
  	return nil
  }
docs: // Index writes the list of file information to the connected peer device