code: func (g *Graph) orderedRelatives(key *Key, attr, orderBy string, cb func(n *Node) []*Node) ([]*Node, error) {
  	if !g.finalized {
  		return nil, ErrNotFinalized
  	}
  	if err := g.validateKey(attr, key); err != nil {
  		return nil, err
  	}
  	if err := validateOrder(orderBy); err != nil {
  		return nil, err
  	}
  	if n := g.nodes[key]; n != nil {
  		return sortByEdgeOrder(cb(n), orderBy), nil
  	}
  	return nil, nil // no node at all -> no related nodes
  }
docs: // orderedRelatives is a common implementation of Children and Parents.