code: func (s *Store) AddPeer(peer string) error {
  	err := s.raft.AddPeer(peer).Error()
  	if err == raft.ErrNotLeader {
  		err = ErrNotLeader
  	} else if err == raft.ErrKnownPeer {
  		return nil
  	}
  	return err
  }
docs: // AddPeer adds a peer to the raft cluster. Panic if store is not open yet.