code: func (s *UserSource) Record() (interface{}, error) {
  	next := atomic.AddUint64(s.n, 1)
  	if next > s.max {
  		return nil, io.EOF
  	}
  	user := s.ug.Record()
  	user.ID = next - 1
  	return user, nil
  }
docs: // Record implements pdk.Source and returns a randomly generated user.