code: func (s *Schema_Set) Normalize() error {
  	if len(s.Entry) == 0 {
  		return errors.New("set requires entries")
  	}
  	set := stringset.New(len(s.Entry))
  	for _, entry := range s.Entry {
  		if entry.Token == "" {
  			return errors.New("blank token")
  		}
  		if !set.Add(entry.Token) {
  			return fmt.Errorf("duplicate token %q", entry.Token)
  		}
  	}
  	return nil
  }
docs: // Normalize will normalize the Schema_Set, returning an error if it is
  // invalid.