code: func (q Quad) NQuad() string {
  	if q.Label == nil || q.Label.String() == "" {
  		return fmt.Sprintf("%s %s %s .", q.Subject, q.Predicate, q.Object)
  	}
  	return fmt.Sprintf("%s %s %s %s .", q.Subject, q.Predicate, q.Object, q.Label)
  }
docs: // Prints a quad in N-Quad format.