code: func WithOpenStringers(os []OpenStringer) Option {
  	return func(s *Source) {
  		if s.files == nil {
  			s.files = make([]*file, 0)
  		}
  		for _, os := range os {
  			s.files = append(s.files, &file{OpenStringer: os})
  		}
  	}
  }
docs: // WithOpenStringers returns an Option which adds the slice of OpenStringers to
  // the set of data sources a Source will read from.