sequence.go
func (s *FileSequence) InvertedFrameRange() string {
	if s.frameSet == nil {
		return ""
	}
	return s.frameSet.InvertedFrameRange(0)
}
func (s *FileSequence) InvertedFrameRange() string {
	if s.frameSet == nil {
		return ""
	}
	return s.frameSet.InvertedFrameRange(0)
}
// InvertedFrameRange returns a new frame range that represents
// all frames *not* within the current frame range. That is, it
// will create a range that "fills in" the current one.