code: func (n *Node) GetNodeIDByRaftID(raftID uint64) (string, error) {
  	if member, ok := n.cluster.Members()[raftID]; ok {
  		return member.NodeID, nil
  	}
  	// this is the only possible error value that should be returned; the
  	// manager code depends on this. if you need to add more errors later, make
  	// sure that you update the callers of this method accordingly
  	return "", ErrMemberUnknown
  }
docs: // GetNodeIDByRaftID returns the generic Node ID of a member given its raft ID.
  // It returns ErrMemberUnknown if the raft ID is unknown.