code: func (s *set) Iter(f func(*Term) error) error {
  	for i := range s.keys {
  		if err := f(s.keys[i]); err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // Iter calls f on each element in s. If f returns an error, iteration stops
  // and the return value is the error.