sequence.go
func (s *FileSequence) Index(idx int) string {
	if s.frameSet == nil {
		return s.String()
	}
	frame, err := s.frameSet.Frame(idx)
	if err != nil {
		return ""
	}
	path, err := s.Frame(frame)
	if err != nil {
		return ""
	}
	return path
}
func (s *FileSequence) Index(idx int) string {
	if s.frameSet == nil {
		return s.String()
	}
	frame, err := s.frameSet.Frame(idx)
	if err != nil {
		return ""
	}
	path, err := s.Frame(frame)
	if err != nil {
		return ""
	}
	return path
}
// Index returns the path to the file at the given index
// in the sequence. If a frame range was not parsed from
// the sequence, this will always returns the original path.
// If the index is not valid, this will return an empty string.