code: func (d *Downloader) RegisterPeer(id string, version int, peer Peer) error {
  	logger := log.New("peer", id)
  	logger.Trace("Registering sync peer")
  	if err := d.peers.Register(newPeerConnection(id, version, peer, logger)); err != nil {
  		logger.Error("Failed to register sync peer", "err", err)
  		return err
  	}
  	d.qosReduceConfidence()
  
  	return nil
  }
docs: // RegisterPeer injects a new download peer into the set of block source to be
  // used for fetching hashes and blocks from.