code: func (ps *peerSet) BestPeer() *peer {
  	ps.lock.RLock()
  	defer ps.lock.RUnlock()
  
  	var (
  		bestPeer *peer
  		bestTd   *big.Int
  	)
  	for _, p := range ps.peers {
  		if td := p.Td(); bestPeer == nil || td.Cmp(bestTd) > 0 {
  			bestPeer, bestTd = p, td
  		}
  	}
  	return bestPeer
  }
docs: // BestPeer retrieves the known peer with the currently highest total difficulty.